The differential closure of a
given ordinary differential field k is characterized to within (differential)
k-isomorphism as a differentially closed (differential) extension field k of k which is
k-isomorphic to a subfield of any differentially closed extension field of k. It has been
conjectured that, in analogy to the cases of the algebraic closure of a field and the
real closure of an ordered field, the differential closure of any differential field k is
minimal, that is, not k-isomorphic to a proper subfield of itself. The conjecture is
here shown to be false.