The probability of the most sensitive genotypes being eliminated from a population due to a contaminant pulse--genetic erosion--is negatively associated to the within-genotype variation. A sensitive genotype with a small phenotypic variation would be more prone to be lost-a critically sensitive genotype. Furthermore, natural populations inhabiting contaminated sites are usually exposed to several pollutants. Such co- or sequential exposure can have severe effects if at least some tolerant clonal lineages surviving one contaminant are sensitive to the others. Such an inverse relationship coupled with a low within-genotype variation potentially enhances genetic erosion. Accordingly, this study evaluated co-tolerance and the occurrence of clonal lineages critically sensitive to 48-hours lethal exposures of copper, zinc, cobalt, and chromium among eight clonal lineages of the cladocerans Daphnia longispina. Median lethal concentrations (LC50) of each metal were found to have the potential to provoke genetic erosion. Pairwise comparisons of LC50, from the eight clonal lineages, revealed neither negative nor positive correlations (r ≤ |0.56|; p ≥ 0.18), but inversely sensitive clonal lineages were found for all pairs of metals. Therefore, besides having the potential to eliminate critically sensitive clonal lineages in a first intermediately lethal pulse, all tested metals may provoke further losses of clonal lineages in an already genetically eroded population.