Ten villages in or near the Markham Valley, northeastern New Guinea, have provided a sample of 476 males for the ascertainment of glucose-6-phosphate dehydrogenase (G6PD) deficiency and 810 individuals of both sexes for the investigation of the β-thalassemia trait. An extreme heterogeneity was found in the prevalence of both traits when the villages were analyzed separately (from 1.5% to 18.2% demonstrating G6PD deficiency and from 0 to 22.8% evincing β-thalassemia trait). The latter result counters an earlier report from the same region that β-thalassemia trait frequencies correlated negatively with altitude and (presumably) positively with endemicity of malaria. The present findings, based on a more representative sample, do not necessarily invalidate the relationship between malaria and β-thalassemia trait suggested by earlier studies, but they do make clear that correlation in New Guinea, at least, may be complicated by random genetic drift, although other possibilities are discussed. The heterogeneous distribution of G6PD deficiency in the present study confirms similar earlier findings. The importance of sample provenience and anthropological data in the interpretation of differences in the distribution of genetically determined traits is stressed.