This study investigated the effects of nonselective predation (culling) and metal toxicity on the genetic diversity of laboratory populations of the cladoceran Daphnia longispina. Populations, comprising five clones differing in their sensitivity to lethal concentrations of a metal-rich mine drainage effluent, were subjected to all possible combinations of three levels (absent, weak, and strong) of each stressor. Observed population densities were compared to those expected from a life history experiment, and clone frequencies were determined by allozyme profiling. Culling did not affect population density, although clonal diversity was higher than the control. Populations stressed by mine drainage recovered to their initial densities within 12 days, though the most sensitive genotypes disappeared under both weak and strong metal stress levels. Because the surviving resistant clones were shown to be the most sensitive ones to other chemicals (Cd and H+), it is suggested that successive inputs of partially lethal concentrations of different chemicals can lead to the disappearance of the population, even if the time between inputs is large enough to allow density recovery. These results suggest that changes in abundance are not enough to evaluate ecological effects of chemicals on the environment and information on co-tolerance and multiple tolerance within populations is highly valuable to prevent their extinction. Moreover, populations exposed to low levels of both stressors showed clonal diversity levels identical to controls, highlighting the importance of low level effects of nonselective stressors in the maintenance of high clonal diversity levels in Daphnia populations, and thus further supporting the intermediate disturbance hypothesis.