Isolated stage III forespores of Bacillus megaterium ATCC 19213 in aqueous suspensions were nearly as dehydrated as mature spores, as indicated by low dextran-impermeable volumes of ca. 3.0 ml per g (dry weight) of cells compared with values of ca. 2.6 for mature spores and 7.3 for vegetative cells. The forespores lacked dipicolinate, had only minimal levels of calcium, magnesium, manganese, potassium, and sodium, and were more heat sensitive than vegetative cells. The effective homogeneous conductivities and dielectric constants measured over a frequency range of 1 to 200 MHz indicated that the inherent conductivities of the forespores were unusually low, in keeping with their low mineral contents, but that the forespores could be invaded by environmental ions which could penetrate dielectrically effective membranes. Overall, our findings support the view that the dehydration of a forespore during stage III of sporogenesis may be the result of ion movements out of the forespore into the sporangium.