Mosquito control applications of fenthion by aerial thermal fog equipment were studied at two sites in Collier County, FL, for sprays that occurred on 20 and 23 June 1984. Acute, lethal effects of fenthion deposited in these estuarine habitats were assessed for caged pink shrimp (Penaeus duorarum), mysids (Mysidopsis bahia), and sheepshead minnows (Cyprinodon variegatus). At Site 1, along a bay with substantial dilution and tidal mixing, fenthion concentrations of 1.5 micrograms/l and 0.29 micrograms/l were recorded immediately after both sprays. Concentrations decreased less than or equal to 0.020 micrograms/l 12 h post-spray and no mortality was observed for caged pink shrimp and mysids. Fenthion concentrations in surface waters were toxic to caged pink shrimp and mysids after both sprays.