The reproductive biology, in response to temperature, season and copper, of a coastal population of opossum shrimps (Crustacea: Mysidacea) was measured. The reproductive pattern for Praunus flexuosus at Keyhaven (west Solent, southern England) has maximum activity in April and maximum reproductive effort in May/June. Increasing temperature decreases incubation time and the reproductive pattern is adapted to compensate for temperature effects. Reproduction was inhibited with copper exposure. Fertilization of the eggs did not occur in any copper treatment (5, 25, 75, 200 μg l−1 copper added). Brooding females exposed to copper suffered a high abortion rate. The inability to produce broods, appears to be a long-term effect, as specimens previously exposed to copper did not produce any broods after two weeks in seawater controls.