Abstract.1. Competition within and between the larval instars of the yellow fever mosquito,Aedes aegypti, can be measured by its effect on stage duration. In a series of laboratory experiments the relative importance of chemical interference and food exploitation in mediating competition between first and fourth instar larvae was investigated.2. In contrast to the results of three previous studies, I found no evidence that a chemical growth retardant played any part in greatly prolonging the stage durations of larvae in both age classes.3. When competition between the two age classes became important, the relative increase in stage duration wasgreater for first instar larvae than for fourth instar larvae. This result can be reproduced with Gilpin&McClelland's (1979) model of competition by food exploitation alone, providing the range of available food particle size is assumed to be an increasing function of age. An additional, but less important, refinement makes the conversion efficiency of food into larval biomass a decreasing function of age.