Morbidity in schistosomiasis is caused by the host response to schistosome eggs which are deposited in the venous system and carried to the tissues. Pathology is proportional to the number of eggs laid, and it is thus important to understand factors underlying worm fecundity and the extent to which fecundity is reflected by eggs passed in the feces, the measurable indicator of infection intensity in humans. More detailed examination of the reported decrease in the fecundity of S. mansoni in immunodeficient scid mice shows that worms are delayed in maturation, but that when mature, they lay normal numbers of eggs.