Caenorhabditis elegans increasingly is attractive as a toxicity test organism, particularly as a model system to study mechanisms of toxicity at a molecular level and the way that these lead to whole organism and population level effects. Inhibitions of growth, reproduction, movement, and feeding rate all have been proposed as sublethal toxicity endpoints. These endpoints are more sensitive than 24-h acute toxicity endpoints, but assays are much more time consuming, making them difficult to use in mass screening. The RNA/DNA ratio, after 48-h exposure to metals, has median effective concentration (EC50) values of 0.05, 0.6, 6.1, and 35 mg/L for Cu, Pb, Cd, and Zn, respectively. This makes it a slightly more sensitive toxicity endpoint than reduction of individual growth after 72-h exposure to the same concentrations. This facilitates the near-simultaneous assessment of sublethal toxicity in many nematode samples. The constant cell number of C. elegans means that different stages in the life history have very different RNA/DNA ratios even in the absence of toxins. So, RNA/DNA ratios can be used only on prereproductive, age-synchronized cultures. Assessing the sublethal toxicity of metals to C. elegans shows that it is sensitive particularly to Cu.