We have sequenced regions of the ZFY and Sox genes in the turtle Chelydra serpentina, a reptile with temperature-dependent sex determination. The ZFY gene in mammals encodes a transcription factor with multiple zinc fingers that may be involved in spermatogenesis as well as other processes. The turtle homologue, (\mathrm{Zft}), is (92 %) identical to the ZFY gene at the nucleotide and amino acid levels in the region of zinc fingers 7-12. There are several Sox genes in the turtle that are only (57-70 %) identical at the nucleotide level and about (55 %) identical at the amino acid level to the human sexdetermining SRY gene. However, the turtle Sox genes, termed TSox, have the conserved motif called the HMGbox (for high mobility group DNA-binding protein) that defines a probable DNA-binding region, and thus are in the same gene family as the Sox genes of other organisms from Drosophila to man. One TSox sequence is identical at the amino acid level to a sequence found in birds, and is (98 %) identical to a sequence encoded autosomally in mouse and in man. The extent of sequence conservation among the Sox genes suggests that some of their functions may be conserved. Phylogenetic analysis of available Sox sequences including SRY (Sry) sequences suggests that there was a high degree of divergence between any possible immediate common ancestor of the turtle Sox sequences and the SRY (Sry) sequences. c 1994 Academic Press, Inc.