Previous cytogenetic analysis of populations of Anopheles marshallii from localities in Zimbabwe and South Africa has revealed the existence of three geographically widespread species within the taxon. A further study, reported here, of individual females from Kosi Bay in Northern Natal has now uncovered another chromosomally distinct population. It is probable that this population represents a fourth species which is closely related to species B and C of the complex. The sex chromosome banding pattern of this new population is presented and its phylogenetic relationship to other species of the complex is considered.