The D. carinata complex includes 9 spp. which are morphologically and genetically very similar yet several species coexist in many Australian freshwater habitats. A measure of reproductive rate, standard egg production (S.E.P.), which was independent of population size structure was introduced. Strong correlation in variation of S.E.P. values for cohabiting species probably indicated strong resource overlap. Weak correlation indicated either resource or physiological differentiation. S.E.P. values were monitored for 17 mo. at 18 sites where D. carinata and D. cephalata cohabited. Although populations were generally resource limited, strong resource overlap was evident. Coexistence depended primarily on seasonal change in relative fitness of the 2 spp. The S.E.P. data for 5 other pairs of species supported the conclusion that resource overlap was general among species in the D. carinata complex. The value of S.E.P. correlation analysis in the assessment of resource overlap and of competition within zooplankton communities was considered.