The nematodeRhabdias bufonisis an anuran lung parasite with a heterogonic life cycle, characterized by alternation of gonochoristic (free-living) and hermaphroditic (parasitic at late larval and adult stages) generations. Adult parasites containing unhatched embryos were isolated from infected amphibians, but all other stages were cultured under controlled laboratory conditions with bacteria on agar plates. Development was studied with particular reference to a comparison of the two generations. No deviation from the strict heterogonic life cycle could be induced by alterations in the conditions of culture. The early embryonic lineages in the two generations are similar but not identical. Early development in both generations takes approximately twice as long as that inCaenorhabditis elegans, and the sequence of divisions is more variable. Nuclei counted in late embryos were similar in number (mean numbers: free-living generation 502; parasitic generation 570). Their number increases only moderately in the free-living generation, reaching a mean of 873 for non-gonadal tissue in adults, similar to that (816) inC. elegans.In contrast, parasitic adults contain in total approximately 4700 somatic and of these 2600 intestinal nuclei—about five times and one hundred times, respectively, the numbers present in free-living adults. We conclude that the dramatic differences in body size and cell numbers between the two generations can be attributed mainly to additional gut tissue-specific cell divisions during the late larval or adult stages of development of the parasitic generation.