SummaryThe values of the coefficients of inbreeding for sex‐linked genes have been calculated for each one of the subtypes of first‐cousin once removed, and second‐cousin marriages.Data are presented on the incidence of the different subtypes of uncle‐niece, aunt‐nephew, first‐cousin, first‐cousin once removed, and second‐cousin marriages in a number of Brazilian populations. The genetic implications of the findings are inferred and discussed.An analysis is made of the mating system, widely known in ‘primitive’ societies, which favours cross‐cousin marriages and disapproves parallel cousin marriages.We are highly indebted to the ecclesiastical authorities who made their files available for analysis. Our thanks are also due to Dr R. Deraemaeker for allowing us to present his unpubl