Perhaps 20% of known animal species are haplodiploid: unfertilized haploid eggs developinto males and fertilized diploid eggs into females. Sex determination in such haplodiploid species does not rely on a difference in heteromorphic sex chromosome composition but the genetic basis has been elucidated in some hymenopteran insects (wasps, sawflies, ants, bees). In these species, the development into one sex or the others depends on an initial signal whether there is only one allele or two different alleles of a single gene, (...) thecomplementary sex determiner(csd), in the zygotic genome. The gene has been most‐recently identified in the honey bee and has been found to encode an arginine serine‐rich (SR) type protein. Heterozygosity generates an active protein that initiates female development while hemizygosity/homozygosity results in a non‐active CSD protein and default male development. I will discuss plausible models of how the molecular decision of male and female is made and implemented. Comparison to hierarchies of dipteran insects suggests that SR‐type protein has facilitated the differentiation of sex‐determining systems and hierarchies. BioEssays 26:1131–1139, 2004. © 2004 Wiley Periodicals, Inc. (shrink)