Abstract: Two subspecies of Hill Mynah (Gracula religiosa) occur in Thailand, the northern race (G. r. intermedia) and the southern race (G. r. religiosa). The southern race is larger than the northern race and has two separate patches of yellow wattle on the ear-coverts, whereas the northern race has a yellow connection between the anterior and posterior wattles. The variation in morphology was investigated by measuring eleven external morphological characteristics of 749 live birds from different habitats throughout Thailand. Birds from different latitudinal zones of Thailand were studied in five equal-interval zones from 5° 30` to 20° 30` N. A descriptive analysis of morphological variables for birds from each latitudinal zone indicated that the average head area, bill size, body size, wing length and tarsus length increased toward the south. One-way analysis of variance among the zones for all morphological parameters showed significant F-ratios. All morphological measurements were negatively correlated with latitude except for tail length which had a positive coefficient. Comparison of morphological variation with climatic variation especially with relative humidity suggests that climatic adaptation may account for subspecific variation in the morphology of this species.