A powerful approach for identifying mammalian primary (gonadal) sex determination genes is the molecular genetic analyses of sex reversal conditions (that is, XX individuals with testicular tissue and XY individuals with ovarian tissue)^1–4. Here we determined the number and chromosomal location of autosomal and X-linked genes that cause sex reversal in C57BL/6J (B6) mice carrying a Y chromosome of Mus domesticus poschiavinus origin (Y^pos). B6 XY^POS mice develop either as females with exclusively ovarian tissue or as true hermaphrodites with ovarian and testicular tissue. In contrast, the Y^POS chromosome is fully masculinizing on most other inbred strain backgrounds. B6-Y^POS sex reversal appears to result from the incompatibility of the Sry (sex determining region, Y chromosome) allele carried on the Y^POSchromosome⁵ with B6-derived autosomal or X-linked loci^6–9. We found strong evidence for the location of one gene, designated tda1 (testis-determining, autosomal 1), at the distal end of Chromosome (Chr) 4 and a second gene, tda2, in the central region of Chr 2. A third gene, tda3, on Chr 5 is implicated, but the evidence here is not as strong. We suggest that B6 alleles at these loci predispose XY^POS fetuses to ovarian tissue development, but no single locus or combination of loci is necessary and sufficient to cause sex reversal. The IDA proteins may regulate Sry expression or form complexes with the SRY protein to regulate other genes, or the tda genes may be activated or repressed by the SRY protein.