1. Laboratory populations of cloned Daphnia pulex at three population phases (growing phase, density peak, declining phase) were exposed to the insecticide carbaryl at 15 μg L−1, which killed juveniles but not adults.
2. The population decline was largest and recovery was slowest when the chemical was applied at the density peak, indicating that the population was most sensitive to the chemical at that stage. Populations in the declining phase were least affected by the treatment.
3. We propose that food condition, which varies with population density, was a major factor affecting vulnerability to carbaryl. In that case, Daphnia populations in lakes would be most vulnerable to toxic chemicals during food-limited condition such as the spring clear-water phase.