The effect of phorbol ester tumor promoters on the development and behavior of a free-living soil nematode, Caenorhabditis elegans, was studied. When young developing C. elegans were grown on E. coli-seeded agar with low concentrations (0.1 microgram/ml) of 12-0-tetradecanoyl-phorbol-13-acetate or phorbol-12,13-didecanoate, their growth was arrested. These tumor promoters reduced the brood size when gravid adults were treated and caused uncoordinated movement in animals treated at any stage of development. The effects of these tumor promoters on nematode development and behavior were partially reversible. The nonpromoting derivatives phorbol and 4 alpha-phorbol-12,13-didecanoate showed no effect on the animals.