Minimal isogeny between Drinfel'd modules

We give a bound for the degree of a minimal isogeny between two Drinfel'd m odules. This result is an anlogue of a theorem first proved on elliptic cur ves and then extended to abelian varieties by Masser and Wustholz. This upp er bound, as in the abelian case depends only on the height of one of the m odules and on the degree of a field over which both modules are defined. We get a polynomial bound.