A floating briquette formulation was developed to allow slow releasing which is useful for the spreading of Bacillus thuringiensis var. israelensis (B.t.i.) into water. A semi-field confined assessment was conducted to investigate the effectiveness mosquito control using the briquettes (37.5% AI) against Anopheles sinensis, Culex pipiens, and Ochlerotatus togoi in small plots. The treatment produced continuously 100% larval reductions against An. sinensis and Cx. pipiens at an over 2.0g briquette until 20 and 25 days, respectively. After 25 days from the treatment, the mortality rates of An. sinensis were 55% and 80% at 4.0g and 8.0g briquettes, respectively. After 30 days from the treatment, the mortality rates of Cx. pipiens were 71% and 86% at 4.0g and 8.0g briquettes, respectively. The mortality rates of Oc. togoi were 99% at an 2.0g briquettes after 20 days, with killing of 75% and 99% at 4.0 and 8.0g briquettes after 30 days, respectively. In field, mosquito control evaluations of the briquettes were performed in four marshes with sewage effluent waters, Busan during the period of June through September, 2002. Culex pipiens larvae were treated with the B.t.i. briquettes at the rate of a briquette per $4m^2$ in marsh. Each treatment of the briquettes resulted in satisfactory level of mosquito control up to 21-33 days. The treatment made the mosquito reduction in total averages of 91.7%, 91.3%, 82.4% 82.0%, and 100.0% in 1, 2, 3, 4 and 5 weeks probably due to be long released period of the briquette.