A structure theorem is given
which characterizes abelian groups which are ultrapowers with respect to
ω-incomplete ultrafilters. It is also proved that any nonprincipal ultraproduct of
abelian groups over a countable index set—or, more generally, with respect to a good
ultrafilter—is an ultrapower with respect to an ω-incomplete ultrafilter. The results
of this paper provide a solution to a problem of L. Fuchs.