The effectiveness of a 10.6% chlorpyrifos controlled-release formulation against Psorophora confinnis (Lynch-Arribalzaga) larvae was determined by in-pool bioassays conducted in a rice culture habitat. Average mortalities over an 11-week test period were 22, 58, 79, and 99 percent in plots treated at 0.25, 0.50, I.0, and 2.0 ppm, respectively, based on a theoretical total initial release of all active ingredient. Chlorpyrifos residues recovered from test plot water averaged 0.0004, 0.0006, 0.0009, and 0.0014 ppm in plots treated at 0.25, 0.50, I.0 and 2.0 ppm, respectively. Based solely upon effectiveness (larval mortality exceeded 95% for 11 weeks at the 2.0 ppm treatment level), and residues recovered from the test plot water (not exceeding 0.0022 ppm at the highest dosage), the controlled-release formulation of chlorpyrifos was considered superior to those larvicide formulations presently available.