The haplodiploid sex determination mechanism in Hymenoptera (males are haploid, while females are diploid) plays an important role in the evolution of this insect order. The sex of hymenopterous insects is determined by a single locus. Females are usually heterozygous and males hemizygous. Under inbreeding conditions, homozygous diploid and sterile males occur, which forms a genetic burden on a population. The discovery of csd gene in honeybees may offer a molecular basis for further understanding of complementary sex determination. In this paper, we discuss about the molecular basis, taxonomic distribution of sl-CSD, and the mechanisms.