In Xenopus, the plasma membrane of the unactivated egg is receptive to sperm only in the animal hemisphere (R. Grey, M. Bastiani, D. Webb, and E. Schertel, 1982, Dev. Biol. 89, 475-487). The reinsemination experiments of investment-free eggs reported in this paper demonstrate that functional sperm entry is lost after activation. Supernumerary sperm were excluded even though the fertilization envelope was absent and the membrane potential had returned to the level found in the unfertilized egg. Even when the electrical block to polyspermy was suppressed by 40 mM NaI (which reduces the membrane potential), polyspermy could be induced only if denuded eggs were initially inseminated in this medium. We estimate that the loss of functional sperm entry, independent of the electrical block, occurs during the first 10 min following fertilization. Sperm readily adhere to the surface of the animal hemisphere of unactivated eggs divested of their extracellular coats, but they do not adhere to the surface of activated eggs. Denuded eggs also adhere to each other, with the surface of the animal hemisphere of unactivated eggs exhibiting the greatest degree of adhesivity. We used electric field-induced fusion (EFIF), without prior dielectrophoresis, to quantify the regional and temporal adhesiveness of eggs. At electric field strengths greater than 8 V/cm, the probability of fusion during EFIF is highest with the animal hemisphere of unactivated eggs, moderate with both the vegetal hemisphere of unactivated eggs and the animal hemisphere of activated eggs, and lowest with the vegetal hemisphere of activated eggs. When pairs of eggs are constructed with different hemispheres in contact, the fusion characteristics of the pair are similar to the more adhesive member of the pair. The regional and temporal differences in the adhesiveness of the Xenopus egg surface correlate with its receptivity to sperm and could possibly account for the plasma membrane's activation-induced loss of functional sperm entry.