Abstract Three studies were conducted to examine the function of ecdysteroids in the development of parasitic nematodes. Ecdysone and 20-hydroxyecdysone were extracted, separated chromatographically, and measured in the reproductive tracts of adult female Ascaris suum. Perienteric fluid and the body wall did not contain measurable levels of these steroids. Levels of 20-hydroxyecdysone were correlated with the third and fourth molts of larvae grown in vitro from the third stage. In a bioassay, addition of ecdysteroid extracted from the female reproductive tract or synthetic ecdysteroid increased the proportion of third-stage larvae that molted after 4 days in culture. This evidence supports the role of ecdysteroids in molting in A. suum, as well as suggesting a function in gametogenesis and embryogenesis.