The relationship between ostracods body size and altitude was studied from 117 aquatic habitats of Mersin province (Turkey). 72.6% of the relationship between 12 ostracods and five environmental variables was explained. Water temperature and pH were the two most effective variables on the species. The mean length of right and left valves of the species were significantly different (p < 0.01, N=3980) at all altitudinal ranges. A strong tendency of changes in valve height with increasing altitude seems to be more prominent than the changes in length for some species.