The three types of intraspecific histocompatibility in ascidians ‐fusibility, allograft rejection or retention, and cell contact reaction with mutual lysis ‐ are reviewed and compared under both genetic and histological profiles. The differences between solitary and colonial species are outlined. The genetics of fusibility in different populations ofBotryllus schlossenis reconsidered with special reference to its bearing on sexual incompatibility, also in the light of new data. The different modes of allorejection are discussed with regard to the cellular and humoral factors involved. Some histological data on nonfusion reactions inB. schlossertare presented. The difficulties of finding a unifying model for the three types of ascidian alloreactivity are outlined.