Residual effectiveness of plaster matrix formulations of temephos and temephos plus an emulsifier (Atlox 3409F) were evaluated against Aedes aegypti larvae during an 8-wk period. Formulations of 2% and 5% (AI) Atlox with 5% (AI) temephos yielded 100% larval mortality for 8 wk. A 5% temephos formulation without the addition of the emulsifier produced 100% larval mortality through only 1 wk.