: The side-effects of permethrin (20% EC) as a Simulium larvicide on aquatic invertebrates and fish was studied under operational vector control conditions to contribute to the evaluation of the product for its possible adoption by the Onchocerciasis Control Programme in West Africa. After 15 weekly applications of the formulation at 0.045 litre per m(3) of water discharge, drift samples virtually contained no Ephemeroptera. Saxicolous fauna density and proportional diversity were also affected. However, both drift and saxicolous fauna recovered to almost pretreatment levels a month after treatment was terminated. Fish showed some evidence of stress but remained in the active treated zone to make trends of catches in the area comparable with those of the control station. The treatment did not result in fish mortalities, and the condition of fish before and after the experimental period was significantly unchanged. Thus, operational use of permethrin by the Programme would not be expected to have permanent adverse effects on the non-target fauna.