none. Diploid males are known to occur in several braconid and ichneumonid species. These diploid males are the result of a single-locus, sex-determination mechanism. Heterozygotes at this sex locus develop into females, whereas hemizygotes (haploids) and homozygotes (diploids) develop into males. Diploid males have a low fertility and their frequency drastically increases with small populations or inbreeding. The implications of this sex-determining mechanism for the use of parasitoids in biological control are explored. Production of diploid males leads to male-biased sex ratios and can reduce rates of establishment and population growth. Taxa in which a single-locus sex determination has been found (e.g., Ichneumonidae and Braconidae) often experience extreme male-biased sex ratios in mass rearing and have been more difficult to establish than taxa with other modes of sex determination (e.g., Chalcidoids). Methods of rearing and release that can enhance the number of sex alleles and the establishment of parasitoids are discussed.