1. Hermaphroditic and gonochoric reproduction are essentially different reproductive strategies that may lead to diverging population responses to adverse environmental conditions. Each strategy implies different physiological mechanisms, which affect life-history traits and represent different ways of dealing with stress. 2. We studied the performance of hermaphroditic vs gonochoric strains in the nematode Caenorhabditis elegans exposed to cadmium stress at the individual and population level. 3. Under control conditions, the gonochoric strain started reproduction earlier than the hermaphroditic strain at a smaller size. This was due to an earlier switch from sperm to oocyte production triggered by male sperm availability. Under cadmium stress hermaphrodites showed a decrease in the size at onset of reproduction, presumably as a strategy to maintain a high population growth rate. In contrast the body size of gonochoric nematodes was not affected. 4. A process-based model (DEBtox) was used as a tool for analysing life-history data and calculating population growth rates. The model fitted the data well using physiologically relevant parameters such as ageing, survival or reproduction related parameters. The simultaneous fit of all life-history traits was used to obtain populations growth rate estimates. 5. The differences between the two C. elegans strains were reflected at the population level. Lower population growth rates, as calculated by DEBtox, were found in the gonochoric strain, largely determined by the proportion of males in the offspring. 6. From the overall results we suggest that the differences found between both populations are due to the reproductive strategy. Under control conditions, CB strain (with gonochoric reproduction) does not favour population growth rates in the short term due to faster ageing and copulation costs on survival. Furthermore, in response to stress this strain also showed lower performance than the N2 hermaphroditic strain, mainly due to a higher sensitivity of survival to the stressor. © 2005 British Ecological Society.