diff --git a/src/EitherT.ts b/src/EitherT.ts
index a5c86e912..c424f41e8 100644
--- a/src/EitherT.ts
+++ b/src/EitherT.ts
@@ -324,12 +324,15 @@ export function chain<M>(
 export function flatMap<M extends URIS2>(
   M: Monad2<M>
 ): <ME, E, A, B>(ma: Kind2<M, ME, Either<E, A>>, f: (a: A) => Kind2<M, ME, Either<E, B>>) => Kind2<M, ME, Either<E, B>>
+/** @internal */
 export function flatMap<M extends URIS>(
   M: Monad1<M>
 ): <E, A, B>(ma: Kind<M, Either<E, A>>, f: (a: A) => Kind<M, Either<E, B>>) => Kind<M, Either<E, B>>
+/** @internal */
 export function flatMap<M>(
   M: Monad<M>
 ): <E, A, B>(ma: HKT<M, Either<E, A>>, f: (a: A) => HKT<M, Either<E, B>>) => HKT<M, Either<E, B>>
+/** @internal */
 export function flatMap<M>(
   M: Monad<M>
 ): <E, A, B>(ma: HKT<M, Either<E, A>>, f: (a: A) => HKT<M, Either<E, B>>) => HKT<M, Either<E, B>> {
diff --git a/src/OptionT.ts b/src/OptionT.ts
index 0cb13c079..3f0872c28 100644
--- a/src/OptionT.ts
+++ b/src/OptionT.ts
@@ -519,9 +519,11 @@ export function chain<M>(
 export function flatMap<M extends URIS>(
   M: Monad1<M>
 ): <A, B>(ma: Kind<M, Option<A>>, f: (a: A) => Kind<M, Option<B>>) => Kind<M, Option<B>>
+/** @internal */
 export function flatMap<M>(
   M: Monad<M>
 ): <A, B>(ma: HKT<M, Option<A>>, f: (a: A) => HKT<M, Option<B>>) => HKT<M, Option<B>>
+/** @internal */
 export function flatMap<M>(
   M: Monad<M>
 ): <A, B>(ma: HKT<M, Option<A>>, f: (a: A) => HKT<M, Option<B>>) => HKT<M, Option<B>> {
diff --git a/src/ReaderT.ts b/src/ReaderT.ts
index 976a71914..6945c498e 100644
--- a/src/ReaderT.ts
+++ b/src/ReaderT.ts
@@ -145,9 +145,11 @@ export function chain<M>(
 export function flatMap<M extends URIS>(
   M: Chain1<M>
 ): <R, A, B>(ma: Reader<R, Kind<M, A>>, f: (a: A) => Reader<R, Kind<M, B>>) => Reader<R, Kind<M, B>>
+/** @internal */
 export function flatMap<M>(
   M: Chain<M>
 ): <R, A, B>(ma: Reader<R, HKT<M, A>>, f: (a: A) => Reader<R, HKT<M, B>>) => Reader<R, HKT<M, B>>
+/** @internal */
 export function flatMap<M>(
   M: Chain<M>
 ): <R, A, B>(ma: Reader<R, HKT<M, A>>, f: (a: A) => Reader<R, HKT<M, B>>) => Reader<R, HKT<M, B>> {
diff --git a/src/StateT.ts b/src/StateT.ts
index aead2819a..2750ff3e0 100644
--- a/src/StateT.ts
+++ b/src/StateT.ts
@@ -142,7 +142,9 @@ export function chain<M>(
 export function flatMap<M extends URIS3>(
   M: Chain3<M>
 ): <S, R, E, A, B>(ma: StateT3<M, S, R, E, A>, f: (a: A) => StateT3<M, S, R, E, B>) => StateT<M, S, B>
+/** @internal */
 export function flatMap<M>(M: Chain<M>): <S, A, B>(ma: StateT<M, S, A>, f: (a: A) => StateT<M, S, B>) => StateT<M, S, B>
+/** @internal */
 export function flatMap<M>(
   M: Chain<M>
 ): <S, A, B>(ma: StateT<M, S, A>, f: (a: A) => StateT<M, S, B>) => StateT<M, S, B> {