Eggs of Xenopus borealis usually can not be fertilized by sperm of Xenopus laevis laevis , while laevis eggs do not exhibit such a fertilization block to borealis sperm. But if borealis eggs obtained from the body cavity and thus lacking jelly are enrobed with laevis jelly following transfer to the body cavity of laevis and subsequent passage through the oviduct, fertilization with laevis sperm can occur. Alternatively, laevis eggs enrobed with borealis jelly can not be usually fertilized with sperm of their own species but are still fertilized by borealis sperm. Evidence is presented that suggests that laevis sperm can not penetrate the innermost jelly layer of borealis eggs.