Waterhyacinth [Eichhornia crassipes (Mart.) Solms] grown in screen-enclosed, outdoor aquaria were treated with factorial combinations of 0, 0.75, 1.5, or 3 kg/ha of the experimental growth retardant EL-509 [α-(4-chlorophenyl)-α-(1-methylethyl)-5-pyrimidine-methanol] and zero, one, two, or three pairs of waterhyacinth weevils (Neochetina eichhorniae Warner)/plant, with three replicates of each weevil-number by retardant level combination. When plant growth was evaluated over a time course, significant effects of weevils and retardant were observed. After 127 days, however, waterhyacinth was reduced as weevil numbers increased, but the growth retardant was ineffective without weevils. Weevils appeared to be more effective when used in combination with the retardant, and the combined effects were additive.