Two synthetic pyrethroids and 5 new organophosphate larvicides were evaluated in the field against stagnant and floodwater mosquitoes. The acute effects of some of these larvicides on some nontarget aquatic insects were also assessed. The synthetic pyrethroid FMC-33297 [3-Phenoxybenzyl (+ cis-trans-3 (2.2 dichlorovinyl-2 dimethylcyclopropanecarboxylate] was the most effective, yielding complete control of larvae and pupae of Culiseta inornata Williston at the rate of 0.025 Ib/acre. At this rate, it is also yielded almost complete control of Psorophora confinnis (L-A) in an irrigated hay field. At lower rates (0.005-0.01), this material was as toxic to mayfly naiads and dragonfly naiads as it was to the target mosquitoes. Among the OP larvicides, CGA-15324 [o-(4-bromo-2-chlorophenyl) )-ethyl S-propyl phosphorothioate] was the most effective one. At 0.05 Ib/acre, it produced complete control of Culex tarsalis Coquillett larvae and pupae. At this rate, it also yielded complete control of P. confinnis in irrigated hay pasture. At larvicidal rate (0.05 Ib/acre) this material produced complete mortality of mayfly naiads, but had no marked effect on dragonfly naiads. The other synthetic pyrethroid and some of the OP larvicides studied, showed good to low activity against the target mosquito species, producing good control at the rates of 0.025-0.I Ib./acre. Most of these possessed high level of toxicity to mayfly naiads, but showed lower toxicity to dragonfly naiads.