  If $R$ is a ring with 1, we call a unital left $R$-module $M$ co-Hopfian
(Hopfian) in the category of left $R$-modules if any monic (epic) endomorphism
of $M$ is an automorphism. For commutative Noetherian $R$ we use results of
Matlis to show that in a certain context every submodule of a co-Hopfian
injective module is co-Hopfian. For these same $R,$ we characterize when a
finitely generated co-Hopfian module has finite length. We describe the
structure of Hopfian and co-Hopfian abelian groups whose torsion subgroup is
cotorsion.
