Sex determination in Drosophila depends on X-linked 'numerator' genes activating early Sex-lethal (Sxl) transcription in females. One numerator gene, sisterless-b (sis-b), corresponds to the achaete-scute (AS-C) T4 basic-helix-loop-helix (bHLH) gene. Two other closely related AS-C bHLH genes, T3 and T5, appear not to function as numerator elements. We analyzed endogenous AS-C expression and show that T4 is the major AS-C numerator gene because it is expressed earlier and more strongly than are T3 and T5. Only T4 expression is detectable during the early syncytial stages when Sxl state is being determined. Nevertheless, the effects of ectopic AS-C gene expression show that T3 and T5 proteins display weak but significant feminizing activities, enhancing male-lethality, and rescuing the female-lethality of sis mutations. Detailed examination of Sxl expression in rescued embryos suggests that female cells may be viable in the absence of detectable Sxl protein expression.