Partulid land snails of the genus Samoana inhabiting the Marquesas Islands of French Polynesia exist as two distinct types, the 'thick-shelled' and 'thin-shelled' species. The two types differ in size, shell-thickness, pigmentation of the shell and mantle, stickiness of the mucus, and length of the tentacles. A study of variation in allozymes indicates that the species form a monophyletic group, within which the differences between the two types have twice evolved independently, once in the northern islands and at least once in the south. In the Society Islands the two suites of characters are found in both Partula and Samoana, where there is evidence of at least four other independent derivations of the two types. The thin-shelled species consistently tend to occur at the higher altitudes (above 200 m). These cases of parallel evolution argue strongly that the complexes of associated characters have arisen through natural selection. (C) 2000 The Linnean Society of London.