Bacillus sphaericus was evaluated in laboratory bioassays against larvae of 7 Arkansas mosquito species and in rice field plots against a single mosquito species. Laboratory-tested species included Aedes aegypti, Ae. albopictus, Ae. vexans, Anopheles punctipennis, Culex quinquefasciatus, Cx. restuans, and Psorophora columbiae. Field plot bioassays were evaluated against Ps. columbiae only. The laboratory formulation tested demonstrated activity against all 7 species. Mortalities were observed at 24 and 48 h posttreatment. Twenty-four-hour LC50 results ranged from 0.84 to 15.04 international toxic units (ITU)/ml for all species and 48-h LC50 results ranged from 0.38 to 9.27 ITU/ml. Twenty-four-hour LC90 results ranged from 1.90 to 48.21 ITU/ml for all species, whereas 48-h LC90 results ranged from 1.30 to 33.83 ITU/ml. Subsequent field plot evaluations yielded control at both dosages (0.45 and 0.9 g/ ha) during the initial 48-h exposure period. Mortalities at 48 h posttreatment for both dosages were 87 and 90%, respectively. Posttreatment mortalities never exceeded 16.7% after 24 h and none were significantly different from control mortality (P < or = 0.05).