Two freshwater mesocosms (10 and 20 m3) were sprayed with lindane and deltamethrin, respectively. We determined the time-dependent changes in pesticide concentration in water, sediment, macrophyte and gastropod samples and the effects on phytoplanktonic, periphytic and insect communities. Lindane residues persisted 19 weeks in water, 18 weeks in sediment, 14 weeks in macrophytes and 13 weeks in gastropods. Lindane had only transitory effects on periphytic and insect communities, resulting in a slight increase in periphyton ash-free dry weight and a moderate reduction of the number of captured insects. Deltamethrin residues persisted in water for only 96 h. Residues were detected in the macrophyte samples for 5 weeks after treatment but never in the sediment and only 24 h after treatment in the gastropod samples. The phytoplanktonic and periphytic communities were positively affected by the treatment whereas emerging insect abundance was strongly reduced. The presence of the control mesocosms as a source of fauna significantly enhanced the re-establishment of the insect community in the treated mesocosms.