Selection pressure applied to larvae of a field population of Aedes aegypti from the Villa Palmeras, Santurce section of San Juan, Puerto Rico resulted in the development of resistance to fenthion in the laboratory. After 9 filial generations of selection pressure, resistance occurred by a factor of 11 compared with the parents, by a factor of 18 compared with the Arecibo, Puerto Rico laboratory strain, and by a factor of 37 compared with the susceptible Fort Detrick laboratory strain. The LC50 values in parts per million were as follows: Fort Detrick laboratory strain, 0.004; Arecibo laboratory strain; 0.008; villa Palmeras parents, 0.013; and villa Palmeras selection F9, 0.15.