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The ovalocytosis polymorphism in New Guinea has hitherto been held to have a mainly coastal distribution, and to be absent from the deep interior of the island. The historical/linguistic studies of Wurm et al. do not suggest any single migration which was exclusively coastal and which linked the northern and southern coasts, but do provide strong evidence of interchange between northern and southern peoples across the narrowest part of the central mountain massif, the Sepik/Fly Divide. Populations sampled along this route, which lies just to the east of the western border of Papua New Guinea, all included people with the ovalocytosis trait; the responsible gene reaches polymorphic frequencies in all of them. This appears to indicate that the polymorphism attained its present distribution not by coastal spread but by extension from the north coast to the south across the Sepik/ Fly Divide. Higher frequencies in the coastal areas than in the interior could have resulted from selection if, as has been suggested, ovalocytosis affords some protection against malaria.