Alloimmune memory, revealed by consistently accelerated second-set graft injection in contrast to unrelated third-party reactions, has been reported in four species of marine sponge. Allogeneic tissue of the marine sponge, Callyspongia diffusa was grafted/parabiosed to examine critically the essential features of alloimmune memory in this member of the least complex major metazoan phylum. Although sponges lack and organized circulatory system, the immunologic memory spreads rapidly through body tissues. Heightened reactivity persisted for only 3 to 4 wk after primary sensitization. This appears to constitute a major difference from the long-term alloimmune memory found among vertebrates. This limited duration of memory, however, is apparently sufficient for the individual sponge to cope with repeated challenges to its integrity.