Experimental Bacillus larvicides designed to float on or near the water surface were compared to labeled standard Bacillus corn-cob-based larvicides using sentinel Anopheles quadrimaculatus larvae in Arkansas rice plots during the 1998 growing season. Experimental floating formulations of Bacillus thuringiensis israelensis applied at 5.58 and 11.18 kg/ha provided up to 100% control of 3rd- and 4th-stage Anopheles larvae within 24-48 h, whereas the water-dispersible granule formulations containing Bacillus sphaericus required 48-72 h to yield >75% mortality in 0.16-ha plots at 11.18 kg/ha. Detecting and targeting the smaller developmental stages (1st- and 2nd-stage larvae) could increase the effectiveness of the tested compounds against An. quadrimaculatus in Arkansas and other rice-growing regions.