Benomile is one of the most used fungicides in the growing fields. In the rainy season brings benomilo with the planktonic communities. The effect of nominal concentrations of benomile under acute and chronic exposure was tested in the cladoceran Ceriodaphnia reticulata that were used. The value of LC(50) was calculated as 2.0 mg L(- 1)+/- 0.15 mg L(- 1) of benomile. In all concentrations, the demographic variables showed inhibition of reproduction. The unique behaviour of Ceriodaphnia reticulata with this particular toxicant is the diminution in the reproduction without affecting the survivorship.