The sex determination systems of Drosophila, birds, marsupial mammals, and eutherian mammals are briefly reviewed. Evidence from poultry polyploids, aneuploids, and gynandromorphs suggests that avian sex is determined by Z to autosome ratio and also that avian sex-limited traits may be under both cell-autonomous and hormonal controls. To identify aneuploids in avian populations, a cDNA clone linked to both the Z and W chromosomes of many avian groups was used to screen large numbers of psittacine birds and identified five birds with atypical patterns of restriction fragments. Two of these birds were examined by laparoscopy and found to be intersexes. These preliminary observations indicate that structural changes in the Z and W chromosomes result in alterations of sex determination.