The presence of fertile queens in M. pharaonls colonies inhibits the appearance of new sexual forms (males and queens). This inhibitory effect is maximal in mature fertile queens, less in young (newly-fertilized) queens, and absent in young alate (virgin) queens. To exert their inhibitory action, queens must have physical contact with other members of the colony. Dead queens and queens rinsed in organic solvents lose their ability to inhibit the pro- duction of sexuals. The Dufour's gland of fertile queen M. pharaonis contains a cast specific chemical which has been identified as 1-isopropenyl- 4,8,12-trimethylcyclotetradeca-3,7,ll-triene (neocembrene). This material is not responsible for the ability of fertile queens to inhibit the production of sexuals, but may serve as a queen- recognition pheromone. Mature fertile queens of M. pharaonis lay about 24 eggs/day, but egg-production is markedly less in young queens and in senile queens. Virgin queens lay few eggs and spend less time on the brood pile than fertile queens. The inhibitory effect of the presence of fertile queens on the production of new sexual forms can be substituted by the artificial introduction of eggs. More- over, queens sterilized by exposure to a juvenile hormone analogue lose their ability to inhibit the production of sexuals. This suggests that workers recognise the presence of eggs as a signal of the presence, and fecundity, of queens. Eggs with the potential to form sexuals are always present in the nest. The caste of female M. pharaonis is determined at an early stage in development, probably in the egg. Workers of M. pharaonis show age-polyethism, and probably use topographical, rather than chemical, cues to recognise the brood stages. Sexual larvae are almost hairless, but worker larvae are covered with rows of bifurcate setae. Workers may use these morphological differences to distinguish between worker and sexual larvae. In queenright colonies of M. pharaonis workers will canniba- lise introduced sexual larvae. By contrast workers in queenless colonies will accept and rear sexual larvae. Thus, in the presence of fertile queens (signalled by the presence of their eggs) workers prevent the appearance of new sexuals by destroying them as eggs or newly-hatched larvae. However, as the egg-producing capacity of the colony declines (e.g. as older queens die), workers will rear, rather than destroy, sexual brood to produce the next generation of sexuals.