Individual and combined effects of larval nutrition and density on developmental mortality, rate of development, adult body size and sex ratio of Culex quinquefasciatus were examined in the laboratory using 4-5 levels of larval food and density. At uniform densities of 1 larva per ml of rearing water or cm2 surface area and with varying amounts of food per larva, the developmental mortality decreased while rate of development and sex ratio (male/total increased, as a linear function of the amount of food per larva. With a constant concentration of food the developmental mortality increased and sex ratio decreased progressively with increasing density and/or decreasing food per larva, but development rate and body size did not show a significant linear change. When larval density was varied with a constant amount of food per larva, the development rate varied as a linear function of the larval density, but developmental mortality, body size and sex ratio, remained unaffected. A bioassay of larval rearing water using conspecific first instar larvae showed that no growth retardant factor (GRF) was produced at larval densities ranging from 1 to 5 per ml or cm2 surface area, when food per larva was not limited. Instead, a growth promoting factor seemed to be produced at higher densities. The results indicate that larval competition for food is more important than competition for rearing space as a population regulating factor in Cx. quinquefasciatus.