A slow-release, briquet formulation of methoprene produced long-term residual activity against Psorophora columbiae larvae in 37.2 m2 (400 ft2) rice plots. An application rate of 1 briquet/9.3 m2 (1/100 ft2) provided significantly (P less than 0.05) greater reduction of adult mosquito emergence (98.2%) than did a rate of 1 briquet/18.6 m2 (1/200 ft2) (89.6%) during 5 insecticidal activity assessments conducted over a period of 58 days. No significant (P greater than 0.05) differences in activity were detected between treated plots which were continuously flooded and those periodically drained and reflooded.