Cypermethrin, a pyrethroid insecticide, was aerially sprayed on a Eucalyptus nitens plantation in northern Tasmania, Australia. Several tributary streams of the Meander River draining the plantation received direct spray drift contamination of the order of 0.05 mg/m2. Increases in invertebrate drift of over 200-times were observed on the day of spraying in Sales Rivulet. Drift was significantly elevated for 8 days after spraying, recovering both in density and relative abundance after early winter floods. Plecoptera and ephemeroptera comprised 89–92% of the drift immediately after spraying, compared with 6–21% pre-spraying and at an uncontaminated site. Benthic abundances of plecoptera and ephemeroptera decreased after spraying in all small streams draining the plantation. Early winter floods were observed to facilitate recolonisation at affected sites. Resident Salmo trutta were collected from the streams before and during 6 months after spraying. Plasma chloride, glucose and protein concentrations were not affected by the spraying event. Significant transient changes in muscle RNA/DNA levels as well as brain and muscle acetylcholinesterase levels and hepatic mixed function oxygenase activity were related to the spraying event. These changes commenced around day 7 and persisted until day 26. Changes in fish diet were also observed, related to the sequence of abundant and depauperate invertebrate drift after spraying. Pathological symptoms in fish were apparently related to dietary intake of cypermethrin from dead and dying invertebrate drift.