A sustained-release, briquet formulation of methoprene (Altosid XR), applied at a rate of one briquet per catch basin in Saginaw, Michigan, provided ca. 70% reduction in emergence of Culex pipiens and Cx. restuans adults, compared with nontreated catch basins, during a period of 15 wk in the summer of 1990. In a parallel study using one briquet per 10.5 liter bucket, there was 99% reduction in adult emergence of these species for a period of 12 weeks. The difference between catch basins and buckets may be attributable to water movement through the catch basins with each rainfall, causing a dilution of methoprene through time. However, both studies indicated that the briquets released methoprene for 12-15 wk, suggesting that this formulation may offer season-long control of Culex mosquitoes from urban catch basins in Michigan, with a single treatment of insecticide.