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SYNOPSIS. The prolific reproductive capabilities of the zebra mussel, Dreissena polymorpha, have facilitated the rapid spread and high densities of this biofouling organism since its accidental introduction into North America less than 10 years ago. Research on its reproductive mechanisms and capabilities may be valuable not only in predicting its further spread, but also in investigating basic mechanisms of reproduction and develop-ment and in developing new strategies to mitigate its impact. Since zebra mussels are dioecious and fertilization occurs externally, coordinated mat-uration, spawning, and other mechanisms have evolved to increase the probability of successful fertilization. The zebra mussel undergoes an an-nual cycle of gonadal growth and gamete maturation, culminating in one or more spawning events in late spring or early summer. Temperature, rates of temperature change, food availability, and effects of neighboring mussels seem to be critical variables that determine reproductive responses. Sero-tonin is a biogenic amine which is implicated in spawning behavior an