Social insects are characterised by a reproductive division of labour. Associated with is the establishment of morphological differing castes (workers, queens). In honeybees, Apis mellifera L., the caste differentiation is most obvious. Caste determination is achieved by differential larval nutrition, a strict environmental factor. Workers are functionally sterile and are able to develop their ovaries and parthenogentically lay eggs. This does not occur in presence of the queen, since queens suppress the ovarial development of workers by a suite of pheromones. Moreover, workers cannibalise worker-laid eggs (worker policing). Reproductive workers develop queen-like phenotypes with respect to physiology and behaviour. These traits are expressed to a high extent in workers of the Cape honeybee, A. m. capensis Esch. Workers of this South African subspecies of the Western honeybee may develop their ovaries rapidly, produce queen-like pheromonal secretions and are able to parthenogenetically lay diploid (female) offspring (= thelytoky). By means of crossing experiments the genetic basis of differing types of parthenogenesis were analysed. A single gene (thelytoky, th) causes thelytokous parthenogenesis, when workers are homozygous for this gene. Using workers of displaying different types of parthenogenesis in bioassays it was shown, that other characters of reproductive dominance (production of pheromones, ovarial development) are affected by the th-gene via pleiotropy. This gene was mapped in the genome using more than 500 microsatellite markers. The gene is located on chromosome 13. Nine candidate genes were identified, one of them a transcription factor (grainy head, grh). Since the allelic condition at the th-gene is crucial for the expression of queen-like characters it seems that there is a strong genetic component in caste determination.