THE mating of the gametes of Ulva mutabilis1 occurs essentially as described for Chlamydomonas2,3 When the gametes are mixed, they cluster and agglutinate with the tips of their flagella. Pairs consisting of a (+) and (−)-gamete leave the cluster, the cell bodies are brought together and fusion initiated. In Chlamydomonas agglutination apparently creates a signal which triggers the later events of cell wall lysis, mating .structure activation and cell fusion4. The gametes of Ulva lack a cell wall5 and have apparently no elaborate mating structure2,5,6. This note shows that the signal in Ulva activates particular areas of the membrane for fusion only for a limited time, and if fusion does not occur then, the ability to fuse is lost permanently.