Aminopterin was applied to the free-living nematode Caenorhabditis briggsae and subsequent growth was recorded. Nematode populations, containing all developmental stages and selected juvenile stages, were exposed to the drug in both growth-promoting and non-promoting media. It is suggested that aminopterin creates a specific requirement for thymine in thymine-free medium. In otherwise growth-promoting medium, aminopterin-induced thymine deficiency will lead to progressively unbalanced growth and maturation and hence to sterility even after removal of the drug. The omission of essential amino acids from the medium during thymine starvation prevents larval growth and results in better reproduction and faster proliferation in aminopterin-free medium. The 4 juvenile stages exhibit a different response to thymine starvation created by aminopterin.