Urban and suburban watersheds have the potential to be highly impacted by chemicals, especially insecticides used to control insect pests on lawns, ornamental plants, and home gardens. Three of the most common lawn-care insecticides detected in urbanized watersheds, carbaryl, chlorpyrifos, and malathion, have been evaluated in the laboratory to determine their acute toxicity, joint-toxicity as mixtures, bioavailability in the presence of organic matter, and effects on survival, growth and development to black fly larvae, Simulium vittatum cytospecies IS-7. An acute orbital shaker toxicity test was used to determine the 48 hr LC50 value of the three insecticides for S. vittatum IS-7 larvae. Results of the 48 hr LC50 tests showed chlorpyrifos to be the most toxic (LC50 = 0.28 g/L) followed by carbaryl (LC50 = 23.72 g/L), and malathion (LC50 = 54.2 g/L). These insecticides were also tested as mixtures using a toxic unit (TU) approach. Toxicity was greater than additive for the ternary mixture of chlorpyrifos-carbaryl-malathion (LC50 = 0.56 TU), and the binary mixtures of chlorpyrifos-malathion (LC50 = 0.72 TU), and carbaryl-malathion (LC50 = 0.78 TU). The binary combination of chlorpyrifos and carbaryl was additive (LC50 = 0.98 TU). Bioavailability of the three insecticides to S. vittatum IS-7 was significantly altered in the presence of a laboratory diet. Chlorpyrifos bioavailability decreased whereas carbaryl and malathion bioavailability increased when diet concentrations were =150 mg/L. Natural seston had little effect on the bioavailability of these insecticides except in carbaryl dosed larvae where mortality was significantly increased when seston concentrations reached 150 mg/L in the flasks. Multiple pulse-exposures of chlorpyrifos, carbaryl and malathion individually and as mixtures in a trough flow-through dosing system had little effect on survival, growth and development of S. vittatum IS-7. Survival in black flies exposed to chlorpyrifos was significantly lower than survival in malathion and carbaryl exposed insects. However, this effect was not significant compared to controls. No significant differences in growth or development were detected between insecticide-exposed and control larvae.