In Xenopus laevis, the fertilization rate increases gradually between the fallopian tube and the ovisac without any marked local rise correlated with a special jelly layer. This suggests that the diameter of the jelly coat, rather than a factor restricted to one or another jelly layer, is important in sperm capacitation in vivo. Xenopus body cavity eggs can be fertilized in vitro after they have passed through the fallopian tube and about a quarter length of adjacent oviduct. The passage time of about 50 min suggests that body cavity eggs in Xenopus are ready for development shortly after ovulation. Single, motile sperm injected into body cavity eggs induce normal development. This indicates that sperm capacitation and membrane interactions between the sperm and the egg are not absolute prerequisites to the transformation of the spermatozoid nucleus into a functional male pronucleus, at least in Xenopus.