In a field trial, pyriproxyfen (Sumilarv 0.5% G) applied at 10 g and 50 g per catch basin and S-methoprene (Altosid XR briquet 2.1%) at 1 briquet per catch basin at a local park provided 100% inhibition of adult emergence (IAE) of the test species, Culex quinquefasciatus, for the first 3 wk. The IAE dropped to 69% to 85% for pyriproxyfen and 68% for methoprene at 4 wk posttreatment. The IAE at 5 to 8 wk posttreatment, although remaining high, was overshadowed by high mortality in control catch basins. The high mortality in control could be attributed to autodissemination of pyriproxyfen by mosquitoes from treated to untreated catch basins.