Hill Mynah Gracula religiosa is a monomorphic bird of which the discrimination of sex is a crucial problem in captive breeding programs. In the past ten years, the wild population of the two subspecies found in Thailand, G.r. intermedia and G. r. religiosa, have drastically decreased due to the demand of the pet market to trade them as talking birds. Sexing Hill Mynah will be an important part in breeding them in captivity. Various methods had been experimented to sex them. Best of all was to obtain sex chromosomes from feather pulp. Male is the homogametic sex (ZZ) whereas female is the heterogametic sex (ZW). To identify Z and W chromosomes in Hill Mynah is undoubtedly an easy task since Z chromosome is nearly twice the size of W chromosome. It is subtelocentric and medium in size of macrochromosomes or the same size as pairs 4, 5, 6 while W chromosome is submetacentric and smaller, the same size as pairs 7, 8, 9. The diploid chromosome number of Hill Mynah was found to be 80. The karyotype consisted of ten pairs of macrochromosomes, nine pairs of autosomes and one pair of sex chromosome and thirty pairs of microchromosomes. Captive breeding of Hill Mynahs in Thailand is no longer perplexed when sex identification is ultimately possible.