Although the California sea hare (Aplysia californica) is well known from neurobiological studies and is raised in the laboratory for this purpose, the life history of this species in the laboratory is less well studied. Therefore we conducted a study of the reproductive period of hatchery-born and -raised A. californica in which sibling animals were reared at stocking densities of 2, 5, 10, 15, and 20 animals per 16-liter cage. Temperature was controlled at 13 to 15 degrees, and the photoperiod was a 14:10-h light:dark cycle. Seawater O2 concentration, pH, and salinity were optimized by seawater flow through the animal cages. Compared with scheduled feedings, an ad libitum algal diet produced early sexual maturity. Despite different growth rates at different animal densities per cage, the age at first sexual maturity (defined as the first egg mass) did not differ significantly among the different densities and averaged 210 +/- 15 days of age. Although there was no difference in the total spawn weight per cage as a function of animal density from 2 to 20 animals per cage, the number of spawning episodes per animal decreased as the cage density increased. The average weight of spawn per animal over its reproductive lifetime decreased as cage density increased. When expressed per day of reproductive maturity, spawn weight per animal decreased as the cage density increased. Finally, larger animals produced more eggs during the 10 spawns at the peak of the spawning period. The largest animals corresponded to the cages containing the fewest animals. Stocking densities between 2 and 20 per cage had little influence on time to maturity and on average number of days of reproductive life. However, stocking density (and thus animal size) strongly influenced so-called lifetime fecundity.