The temperature and field dependence of the magnetic susceptibility of
well-characterized K3C60 and Rb3C60 show that the zero-field-cooled (
ZFC) diamagnetism below T(c) is irreversible for both systems, while t
he field-cooled (FC) curve is reversible, and that the ZFC diamagnetis
m is more than four times larger than the FC. This behavior is attribu
ted to a vortex-glass state below T(c), similar to that observed in hi
gh-T(c) cuprates as well as disordered conventional type-II supercondu
ctors. The field dependence of the vortex glass to vortex fluid transi
tion temperature T is found to follow the de Almeida-Thouless relatio
n H=H-0[1-T(H)/T*(0)]gamma, where gamma = 3/2 for both compounds.