Unlike mammals, birds have a ZZ male/ZW female sex-determining system. In most birds, the Z is large and gene rich, whereas the W is small and heterochromatic, but the ancient group of ratite birds are characterized by sex chromosomes that are virtually homomorphic. Any gene differentially present on the ratite Z and W is therefore a strong candidate for a sex-determining role. We have cloned part of the candidate bird sex-determining gene DMRT1 from the emu, a ratite bird, and have shown that it is expressed during the stages of development corresponding to gonadal differentiation in the chicken. The gene maps to the distal region of the Z short arm and is absent from the large W chromosome. Because most sequences on the emu W chromosome are shared with the Z, the Z-specific location constitutes strong evidence that differential dosage of DMRT1 is involved in sex determination in all birds. The sequence of emu DMRT1 has 88% homology with chicken DMRT1 and 65% with human DMRT1. Unexpectedly, an unexpressed 270-bp region in intron 3 of emu DMRT1 showed 90% homology with a sequence in the corresponding intron of human DMRT1. This extraordinarily high conservation across 300 million years of evolution suggests an important function, perhaps involved in control of DMRT1 expression and vertebrate sex determination.