diff --git a/.eslintrc.js b/.eslintrc.js
index 6998bca453f18..b6d07a79e6601 100644
--- a/.eslintrc.js
+++ b/.eslintrc.js
@@ -99,6 +99,18 @@ module.exports = {
     'react-internal/warning-args': ERROR,
     'react-internal/no-production-logging': ERROR,
     'react-internal/no-cross-fork-imports': ERROR,
+    'react-internal/no-cross-fork-types': [
+      ERROR,
+      {
+        old: [
+          'firstEffect',
+          'nextEffect',
+          // Disabled because it's also used by the Hook type.
+          // 'lastEffect',
+        ],
+        new: ['subtreeTag'],
+      },
+    ],
   },
 
   overrides: [
diff --git a/scripts/eslint-rules/__tests__/no-cross-fork-types-test.internal.js b/scripts/eslint-rules/__tests__/no-cross-fork-types-test.internal.js
new file mode 100644
index 0000000000000..57c50a55b315f
--- /dev/null
+++ b/scripts/eslint-rules/__tests__/no-cross-fork-types-test.internal.js
@@ -0,0 +1,89 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @emails react-core
+ */
+
+'use strict';
+
+const rule = require('../no-cross-fork-types');
+const RuleTester = require('eslint').RuleTester;
+const ruleTester = new RuleTester({
+  parserOptions: {
+    ecmaVersion: 8,
+    sourceType: 'module',
+  },
+});
+
+const newAccessWarning =
+  'Field cannot be accessed inside the old reconciler fork, only the ' +
+  'new fork.';
+
+const oldAccessWarning =
+  'Field cannot be accessed inside the new reconciler fork, only the ' +
+  'old fork.';
+
+ruleTester.run('eslint-rules/no-cross-fork-types', rule, {
+  valid: [
+    {
+      code: `
+const a = obj.key_old;
+const b = obj.key_new;
+const {key_old, key_new} = obj;
+`,
+      filename: 'ReactFiberWorkLoop.js',
+    },
+    {
+      code: `
+const a = obj.key_old;
+const {key_old} = obj;
+`,
+      filename: 'ReactFiberWorkLoop.old.js',
+    },
+    {
+      code: `
+const a = obj.key_new;
+const {key_new} = obj;
+`,
+      filename: 'ReactFiberWorkLoop.new.js',
+    },
+  ],
+  invalid: [
+    {
+      code: 'const a = obj.key_new;',
+      filename: 'ReactFiberWorkLoop.old.js',
+      errors: [{message: newAccessWarning}],
+    },
+    {
+      code: 'const a = obj.key_old;',
+      filename: 'ReactFiberWorkLoop.new.js',
+      errors: [{message: oldAccessWarning}],
+    },
+
+    {
+      code: 'const {key_new} = obj;',
+      filename: 'ReactFiberWorkLoop.old.js',
+      errors: [{message: newAccessWarning}],
+    },
+    {
+      code: 'const {key_old} = obj;',
+      filename: 'ReactFiberWorkLoop.new.js',
+      errors: [{message: oldAccessWarning}],
+    },
+    {
+      code: 'const subtreeTag = obj.subtreeTag;',
+      filename: 'ReactFiberWorkLoop.old.js',
+      options: [{new: ['subtreeTag']}],
+      errors: [{message: newAccessWarning}],
+    },
+    {
+      code: 'const firstEffect = obj.firstEffect;',
+      filename: 'ReactFiberWorkLoop.new.js',
+      options: [{old: ['firstEffect']}],
+      errors: [{message: oldAccessWarning}],
+    },
+  ],
+});
diff --git a/scripts/eslint-rules/index.js b/scripts/eslint-rules/index.js
index 1451eb906d28f..0a08b0d4cfcdf 100644
--- a/scripts/eslint-rules/index.js
+++ b/scripts/eslint-rules/index.js
@@ -8,5 +8,6 @@ module.exports = {
     'invariant-args': require('./invariant-args'),
     'no-production-logging': require('./no-production-logging'),
     'no-cross-fork-imports': require('./no-cross-fork-imports'),
+    'no-cross-fork-types': require('./no-cross-fork-types'),
   },
 };
diff --git a/scripts/eslint-rules/no-cross-fork-types.js b/scripts/eslint-rules/no-cross-fork-types.js
new file mode 100644
index 0000000000000..eaa9f07d74e06
--- /dev/null
+++ b/scripts/eslint-rules/no-cross-fork-types.js
@@ -0,0 +1,126 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @emails react-core
+ */
+
+/* eslint-disable no-for-of-loops/no-for-of-loops */
+
+'use strict';
+
+function isOldFork(filename) {
+  return filename.endsWith('.old.js') || filename.endsWith('.old');
+}
+
+function isNewFork(filename) {
+  return filename.endsWith('.new.js') || filename.endsWith('.new');
+}
+
+function warnIfNewField(context, newFields, identifier) {
+  const name = identifier.name;
+  if (name.endsWith('_new') || (newFields !== null && newFields.has(name))) {
+    context.report({
+      node: identifier,
+      message:
+        'Field cannot be accessed inside the old reconciler fork, only the ' +
+        'new fork.',
+    });
+  }
+}
+
+function warnIfOldField(context, oldFields, identifier) {
+  const name = identifier.name;
+  if (name.endsWith('_old') || (oldFields !== null && oldFields.has(name))) {
+    context.report({
+      node: identifier,
+      message:
+        'Field cannot be accessed inside the new reconciler fork, only the ' +
+        'old fork.',
+    });
+  }
+}
+
+module.exports = {
+  meta: {
+    type: 'problem',
+    fixable: 'code',
+  },
+  create(context) {
+    const sourceFilename = context.getFilename();
+
+    if (isOldFork(sourceFilename)) {
+      const options = context.options;
+      let newFields = null;
+      if (options !== null) {
+        for (const option of options) {
+          if (option.new !== undefined) {
+            if (newFields === null) {
+              newFields = new Set(option.new);
+            } else {
+              for (const field of option.new) {
+                newFields.add(field);
+              }
+            }
+          }
+        }
+      }
+      return {
+        MemberExpression(node) {
+          const property = node.property;
+          if (property.type === 'Identifier') {
+            warnIfNewField(context, newFields, property);
+          }
+        },
+
+        ObjectPattern(node) {
+          for (const property of node.properties) {
+            const key = property.key;
+            if (key.type === 'Identifier') {
+              warnIfNewField(context, newFields, key);
+            }
+          }
+        },
+      };
+    }
+
+    if (isNewFork(sourceFilename)) {
+      const options = context.options;
+      let oldFields = null;
+      if (options !== null) {
+        for (const option of options) {
+          if (option.old !== undefined) {
+            if (oldFields === null) {
+              oldFields = new Set(option.old);
+            } else {
+              for (const field of option.new) {
+                oldFields.add(field);
+              }
+            }
+          }
+        }
+      }
+      return {
+        MemberExpression(node) {
+          const property = node.property;
+          if (property.type === 'Identifier') {
+            warnIfOldField(context, oldFields, property);
+          }
+        },
+
+        ObjectPattern(node) {
+          for (const property of node.properties) {
+            const key = property.key;
+            if (key.type === 'Identifier') {
+              warnIfOldField(context, oldFields, key);
+            }
+          }
+        },
+      };
+    }
+
+    return {};
+  },
+};