Abstract: Avian cDNA nucleotide sequences for use as hybridization probes are provided. Probes derived from the sequences find use in defining the sex of a bird, usually by producing hybridization patterns that are sex-specific for most avian species. For a given avian species the probes may hybridize to both Z and W chromosomes so as to differentiate between the two chromosomes on the basis of restriction fragment length polymorphisms. Alternatively, the probes may hybridize exclusively to one of the two sex chromosomes in some species. The hybridization probes taught are also useful for the isolation of other nucleotide sequences that may be used to generate sex-specific hybridization patterns.