The prevalence of different haemoglobins and their interaction with malaria have been studied in Garki, Kano State, Nigeria. Sickle cell trait was present in 24% of newborn and 29% of those aged over five years. Hb.AC was present in 0.7%. Frequency of both haemoglobin variants was greater in Hausa than Fulani. Sickle cell anaemia was almost invariably fatal in early childhood. The distribution curve of percentage of Hb.S in sickle cell trait subjects was normal, and did not demonstrate any high frequency of a gene for alpha-thalassaemia. The presence of beta-thalassaemia minor could not be tested, but Hb.S/beta-thalassaemia was not detected. Hb.S gene frequency appears to have been maintained by a fitness in heterozygotes of 21% over normal homozygotes; increased fertility and high mutation rate did not make any apparent contribution. Hb.AS subjects had on average lower frequency and considerably lower densities of Plasmodium falciparum trophozoites than Hb.AA from the age of 30 to 59 weeks; density was less in sickle cell trait up to age three years in the dry season only. It is suggested that the survival advantage and hence the prevalence of sickle cell trait may be greatest in some hyperendemic areas and less where malaria transmission is extremely high or when it is high and unvaried.