Copulation in the acoelOtocelis luteolamay result in mutual or one-sided insemination. Fifty specimens cultured with a species ofNaviculawere allowed to mate in pairs for a period of 6 h, 50 were allowed to mate in pairs for 24 h, and nine were allowed to mate freely for 24 h. All wows were then isolated in separate containers that were examined at frequent intervals in order to record the number of eggs and young. The number of progeny produced by worms that had been inseminated ranged from 1 to 65, and one worm had progeny as late as 55 days after mating. This study shows thatO. luteolais able to store viable sperm in its seminal bursa for several weeks.