Effects of the insecticide chlorpyrifos (cpf) on young (<l d old) and adult (7-9 d old) Daphnia pulex and the ability of D. pulex to recover after exposure were tested in the laboratory. Populations of D. pulex exposed to cpf were studied in the laboratory and field. In the field, cpf was applied in a single dose. The effects of that dose on D. pulex populations were tested and compared with the effects of a simulated single dose and its ensuing decay on populations in the laboratory. The lowest no-observed- effect concentration was about 0.05 μg/L (actual concentration). Chlorpyrifos immobilized D. pulex several days before death. Juveniles became immobilized faster than adults. Even when no longer exposed, D. pulex immobilized by cpf died. In the laboratory and field, observed population dynamics and effects of cpf on these dynamics were very similar. The median effective concentration for population size after 2 and 7 d of exposure did not differ between the laboratory and field (2 d, 0.38 and 0.31 μg/L, respectively; 7 d, 0.25 and 0.34 μg/L, respectively). Effects of cpf on single-species Daphnia populations in the laboratory and field can be predicted from effects observed in tests with individuals.