Land snails of the genus Partula Fer. inhabiting the island of Moorea in French Polynesia show complex patterns of partial speciation. The late H. E. Crampton described eleven species from the island. Two have since been relegated to the genus Samoana. Of the remaining nine taxa, as many as four may coexist at a single locality without evidence of hybridization. Nevertheless an allele appearing in any of the taxa is potentially capable of spreading to any of the others, although in the process it may have to pass through several intervening taxa. Large morphological, ecological and behavioural differences have evolved despite this potential for gene flow. The biological differences are not reflected in 20 enzyme loci surveyed by conventional starch-gel electrophoresis, nor in the numbers of chromosomes. It is clear that a significant part of the morphological divergence between taxa has taken place of Moorea. Our observations are consistent with the view that all the Moorean taxa (excepting Samoana) are descendants of a single ancestral invasion, although the possibility of more than one invasion cannot be excluded. The pattern of differentiation shows at least two cases of circular overlap (`ring species'), with diameters of 2 and 3 km, and several apparent examples of character displacement. Coexisting taxa tend to partition the available habitat, although the separation is never complete.