Diflubenzuron (DFB) inhibits growth and development of Culex pipiens L. larvae at 4 ppb. With 42 hr exposure, a period allowing two molts, it causes a dose-dependent reduction in body weight and chitin content, such that at 9 ppb these are only 66 and 27% of control respectively. There is also a dose dependent increase in the instar duration and mortality. Following [14C} glucose feeding. DFB has no effect on the transport of [14C] glucose and its [ 14C] metabolites into the integument of fourth instar larvae, but causes a dose-dependent decrease in their incorporation into the newly-formed chitin.