Effects were assessed on the total biological community in complex laboratory streams caused by continuous exposure to diflubenzuron (1-(4-chlorophenyl)-3-(2,6-difluorobenzoyl) urea). The insect fauna suffered direct toxic effects at concentrations of 1.0 microgram/L and greater. The algal and fungal floras were mildly affected at the same concentrations; apparently indirect effects in response to the reduced herbivore and shredder components of the insect fauna. No effects were observed on the bacteria, oligochaetes or gastropods, at any of the test concentrations. Within the insect fauna, differences in sensitivities were observed: mayflies and stoneflies were affected at 1.0 microgram/L, dipterans were affected at 10.0 microgram/L, and coleopterans were apparently unaffected at any of the test concentrations. Effects on caddisflies could not be determined due to their failure to become established in the stream systems. These differences between insect orders in sensitivity, coupled with differences in generation time, may reduce the overall benefits of the insecticide. For example, if diflubenzuron is used for chironomid control, the result may be more severe and longer lasting effects on nontarget mayfly and stonefly populations.