Sex in the nematode Caenorhabditis elegans is normally determined by the X chromosome to autosome (X:A) ratio, with XX hermaphrodites and XO males. Previous work has shown that a set of at least four autosomal genes (her-1, tra-2, tra-3, and tra-1) is signaled by the X:A ratio and appears to act in a regulatory pathway to determine sex. Twenty-one new recessive alleles of the gene fem-1(IV) (formerly isx-1) have been isolated. Seven of these may be null alleles; one of these is an amber mutation. The other 14 alleles are temperature sensitive. The putative null mutations cause both XO and XX animals to develop as females when the mother as well as the zygote is fem-1(-). Therefore, fem-1(+) is required (a) for the development of the male body and (b) for spermatogenesis in males and hermaphrodites. In addition, fem-1 shows a maternal effect: wild-type fem-1 product partially rescues the development of fem-1(-) progeny. By analyzing double mutants it has been shown that fem-1(+) is part of the sex-determination pathway and has two distinct functions: (1) in the soma it prevents the action of tra-1, thereby allowing male development to occur, and (2) in the germline it is necessary for spermatogenesis in both sexes.