AbstractThough several studies have examined the relationship between parental ABO blood types and reproductive success, results have been inconsistent. In this paper, the relationship is considered using the data collected by Newton Morton and colleagues from a sample of 1,068 nuclear families of migrant workers in Brazil. Morton et al., using segregation analysis, compared parental and offspring generations and found evidence of selection acting through the mechanisms of maternal/fetal incompatibility in the ABO blood group system (Morton et al. [1966] Am J Hum Genet18:153–171). Further analysis of the same sample yields results which favor the conclusions that: (1) type B mothers have, on average, more offspring than mothers of other ABO blood types; (2) selection as expressed in reproductive differences affects females and not males; and (3) the ABO polymorphism in this sample is maintained by favoring reproductive success in heterozygous mothers. © 1995 Wiley‐Liss,