Laboratory and field comparisons were made with two wettable powder formulations of Bacillus thuringiensis serotype H-14 (B. t. H-14) prepared in Czechoslovakia ("Moskitur") and the USSR ("Baktokulicid"). Expressed in the international Aedes aegypti toxic units (TU X mg-1) the potency of these two test formulations was greater than that of the Institute Pasteur Standard IPS-78 (= 1,000 TU X mg-1), i.e. Moskitur had a potency of about 1,500 TU X mg-1 and the Soviet Baktokulicid 2,000 TU X mg-1. The Baktokulicid and Moskitur LC 90 values for laboratory-reared Aedes aegypti larvae were, respectively, 0.11 and 0.16 mg X l-1. The range of LC 90 values for the Czechoslovak wild-caught mosquito species of the genera Aedes and Culex was 0.14-0.31 mg X liter-1 with Moskitur, 0.11-0.41 mg X l-1 with Baktokulicid, and 0.16-0.48 mg X l-1 with IPS-78. The susceptibility of laboratory Anopheles stephensi larvae was close to that of Aedes aegypti, larvae of An. messeae required many times as much Baktokulicid (1.6 mg X l-1) and Moskitur (more than 6.4 mg X l-1) for 90% mortality as did other mosquito species. The aim of outdoor assays was to establish the minimum Moskitur and Baktokulicid rates giving a 100% control of mosquito larvae. For Ae. cantans breeding habitat in flood plain forest areas these rates ranged between 0.1-0.5 mg X l-1 (0.2-1.0 kg X ha-1), for Ae. vexans control on artificially irrigated meadows between 0.8-2.0 mg X l-1 (1.2-3.0 kg X ha-1). Consistently with laboratory bioassays, Baktokulicid gave 100% control of An. messeae 4th instar larvae at a dose as high as 3.2 mg X l-1, Moskitur gave 23.1% kill at 6.4 mg X l-1. The effect of Moskitur and Baktokulicid formulations was immediate, larvae that hatched 7-14 days posttreatment survived. The efficacy of B. t. H-14 outdoor treatments tended to markedly decrease with the larval densities exceeding 100 larvae per 1 dm2. Species of nontarget aquatic organisms, including the Diptera Chaoborus crystallinus, Mychlonyx sp. and Dixidae, were not noticeably affected by treatments with B. t. H-14 formulations used.