In this paper the author reports the results of three months' study of the saltwater-breeding member of the Anopheles gambiae complex of sibling species on Mauritius.There is evidence for the views that this form's distribution on the island is limited by the availability of suitable breeding areas, that it does not usually disperse far from the breeding grounds or coast, and that it is probably not an important vector except, perhaps, in the near vicinity of its breeding places.Some new evidence is presented in support of the view that this form (and forms A and B) are distinct species. This turns on the observed close coexistence of these three forms on Mauritius, supported by a theoretical consideration of what would be expected to happen in such circumstances if a system of random mating prevailed.Evidence is given that the Mauritian saltwater-breeding form of the A. gambiae complex is conspecific with the form occurring on the east coast of Africa.The practical importance of reaching general agreement on the evolutionary status of the members of the A. gambiae complex is emphasized.