A new deficient variant of glucose-6-phosphate dehydrogenase (G6PD) causing severe congenital nonspherocytic hemolytic anemia (CNSHA) is described. The variant enzyme, characterized by slow electrophoretic mobility, extreme in vivo and in vitro lability, high Km for G6P and strongly acidic pH optimum, appears to be unique, and has been designated G6PD Genova. Investigation of an obligate heterozygote using various cytochemical, biochemical and recombinant-DNA techniques showed G6PD mosaicism in the erythrocytes and leukocytes. Therefore, the presence of a disadvantageous mutation at one Gd locus did not determine selection in favor of the normal allele in the heterozygote's hemopoietic cells.