Many theoretical studies have suggested that disruptive selection plays an important role in phenotypic divergence, but few studies have determined the action of disruptive selection on phenotypic divergence via field studies. This study investigated the effect of disruptive selection on shell colour polymorphism in the Japanese land snail Euhadra peliomphala simodae to determine whether extreme phenotypes of snail shell colour are favored over intermediate phenotypes. We conducted field surveys on an oceanic island with black, yellow, and intermediate-coloured E. p. simodaesnails, capturing and marking approximately 1,800 individual snail and monitoring their survival over 18 months. We quantified their shell colours against images and examined the frequency distribution of shell colour variation. The variation exhibited a bimodal distribution with a far lower frequency of intermediate-coloured snails than of black oryellow snails. The population sizes of the three snail groups fluctuated synchronously with the changing seasons. Bayesian estimates showed lower survival rates for the juvenile intermediate-coloured snails than for juvenile black and yellow snails, implying there was disruptive selection associated with shell colour. We suggest this disruptive selection may have resulted in the evolutionary divergence of the snail’s shell colour within the lineage having high shell colour variations.