To evaluate shell thickness and factors affecting it, weights and heights of 2097 Pomacea canaliculata shells were measured. Specimens were obtained from paddy fields and adjacent canals in two areas in Kawasoe-machi, Saga Prefecture, Japan (33°11'N, 130°19'E), in 2004. Logarithm-transformed shell weights and heights revealed a good regression line, and the deviation of actual weight from expected weight of each shell was regarded as a representative of overall shell thickness (the shell-thickness index). Values of the index were larger in shells collected from canals than in those from paddy fields. Overall, shells appeared to be thinner in June and July than in August and November, irrespective of habitats, presumably due to rapid growth of the snail in early summer.