diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4db51a9e86be53f50a9d73ef93906893fa386345..93f22676d2cec61e4dbf5aad00b4a20221e73df1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,8 @@ Under development (0.4.1)
   * Split checking and expansion of export specification into two
     subsequent steps (by Yannik Potdevin, fixes #1335)
   * Consider parenthesized type expressions in the Curry AST (by Katharina Rahf)
+  * Added syntax extension `ExistentialQuantification` that allows the use
+    of existentially quantified types in data and newtype constructors
 
 Version 0.4.0
 =============
diff --git a/src/Checks/SyntaxCheck.hs b/src/Checks/SyntaxCheck.hs
index 1c82dbcd0e83ae4b326889284936bd1096b1ed6c..92c0d035dfabf693bf0a9f29038470da737b4475 100644
--- a/src/Checks/SyntaxCheck.hs
+++ b/src/Checks/SyntaxCheck.hs
@@ -28,7 +28,7 @@ module Checks.SyntaxCheck (syntaxCheck) where
 #if __GLASGOW_HASKELL__ < 710
 import           Control.Applicative        ((<$>), (<*>))
 #endif
-import Control.Monad                      (unless, when)
+import Control.Monad                      ((>=>), unless, when)
 import qualified Control.Monad.State as S (State, runState, gets, modify)
 import Data.List                          (insertBy, intersect, nub, partition)
 import Data.Maybe                         (isJust, isNothing)
@@ -495,15 +495,30 @@ checkDecls bindDecl ds = do
 -- -- ---------------------------------------------------------------------------
 
 checkDeclRhs :: [Ident] -> Decl -> SCM Decl
-checkDeclRhs _   (DataDecl p tc tvs cs) =
-  DataDecl p tc tvs <$> mapM checkDeclLabels cs
-checkDeclRhs bvs (TypeSig      p vs ty) =
+checkDeclRhs _   (DataDecl   p tc tvs cs) =
+  DataDecl p tc tvs <$> mapM (checkConstrDecl >=> checkDeclLabels) cs
+checkDeclRhs _   (NewtypeDecl p tc tvs c) =
+  NewtypeDecl p tc tvs <$> checkNewconstrDecl c
+checkDeclRhs bvs (TypeSig        p vs ty) =
   (\vs' -> TypeSig p vs' ty) <$> mapM (checkLocalVar bvs) vs
-checkDeclRhs _   (FunctionDecl p f eqs) =
+checkDeclRhs _   (FunctionDecl   p f eqs) =
   FunctionDecl p f <$> mapM checkEquation eqs
-checkDeclRhs _   (PatternDecl  p t rhs) =
+checkDeclRhs _   (PatternDecl    p t rhs) =
   PatternDecl p t <$> checkRhs rhs
-checkDeclRhs _   d                      = return d
+checkDeclRhs _   d                        = return d
+
+checkConstrDecl :: ConstrDecl -> SCM ConstrDecl
+checkConstrDecl c@(ConstrDecl _ evs   _ _) = checkExistVars evs >> return c
+checkConstrDecl c@(ConOpDecl  _ evs _ _ _) = checkExistVars evs >> return c
+checkConstrDecl c@(RecordDecl _ evs   _ _) = checkExistVars evs >> return c
+
+checkNewconstrDecl :: NewConstrDecl -> SCM NewConstrDecl
+checkNewconstrDecl c@(NewConstrDecl _ evs _ _) = checkExistVars evs >> return c
+checkNewConstrDecl c@(NewRecordDecl _ evs _ _) = checkExistVars evs >> return c
+
+checkExistVars :: [Ident] -> SCM ()
+checkExistVars []     = ok
+checkExistVars (ev:_) = checkExistentialQuantificationExtension $ idPosition ev
 
 checkDeclLabels :: ConstrDecl -> SCM ConstrDecl
 checkDeclLabels rd@(RecordDecl _ _ _ fs) = do
@@ -1034,6 +1049,10 @@ checkFPTerm _ (InfixFuncPattern   _ _ _) = ok -- do not check again
 -- Miscellaneous functions
 -- ---------------------------------------------------------------------------
 
+checkExistentialQuantificationExtension :: Position -> SCM ()
+checkExistentialQuantificationExtension p = checkUsedExtension p
+  "Existentially quantified types" ExistentialQuantification
+
 checkFuncPatsExtension :: Position -> SCM ()
 checkFuncPatsExtension p = checkUsedExtension p
   "Functional Patterns" FunctionalPatterns
diff --git a/src/CompilerOpts.hs b/src/CompilerOpts.hs
index 9b633b6953181ba905537f31cb7982cde2cacab6..a60231e0d1e4803a0fc65810c39909a71e17c988 100644
--- a/src/CompilerOpts.hs
+++ b/src/CompilerOpts.hs
@@ -233,6 +233,8 @@ extensions =
     , "desugar negated literals as negative literal")
   , ( NoImplicitPrelude , "NoImplicitPrelude"
     , "do not implicitly import the Prelude")
+  , ( ExistentialQuantification , "ExistentialQuantification"
+    , "enable existentially quantified types")
   ]
 
 -- -----------------------------------------------------------------------------