Three field trials were conducted at Guana River State Park, Florida to evaluate the effectiveness of the granule formulation of methoprene, Altosid® XR-G, against salt-marsh mosquitoes. Three applications of Altosid XR-G (1.5% s-methoprene) were made at application rates of 9.0, 4.5, and 2.3 kg/ha. Pupae were collected from control and treated sites after inundation. Under field conditions, Altosid XR-G gave 44.6% control at 67 days posttreatment at Site 1 (9.0 kg/ha), 43.7% control at 67 days posttreatment at Site 2 (4.5 kg/ha), and 38% control at 53 days posttreatment at Site 3 (2.3 kg/ha). All treatments taken together achieved an adjusted 10-wk cumulative mosquito emergence inhibition of 69%. Collections were composed of Ochlerotatus sollicitans 68%, Oc. taeniorhynchus 27%, and Psorophora columbiae 5%.