Field censuses and laboratory experiments show that in the Argentine ant, Iridomyrmex humilis (Mayr), c. 90% of the queens are executed by workers in May, at the beginning of the reproductive season. The reduction in the number of queens probably decreases the inhibition exerted by queens on the differentiation of sexuals and thus allows the production of new queens and males shortly thereafter. In the laboratory, there was no correlation between the percentage of queens executed and their weight or fecundity. At the time of execution of queens, nearly all queens were of the same age; less than 1 year. Therefore it is not likely that the age of queens plays any role in the choice that workers make in the queens they executed. Execution of these queens results in a heavy energetic cost for the colony which amounts c. 8% of the total biomass. This behaviour of workers executing nestmate queens is discussed with regard to possible evolutionary significance at the queen and worker level.