The efficacy of new and frequently used formulations of Bacillus thuringiensis israelensis (B.t.i.) has been evaluated in the laboratory and in the field conditions under which they will be used in Germany. The principles governing the successful introduction of new formulations of microbial control agents into routine programs have been highlighted. The potency of the formulations in use (Teknar, Bactimos, and Vectobac) and their efficacy against the indigenous mosquito species Aedes vexans, Aedes cantans, and Culex pipiens have been assessed to determine the minimum effective dosage in the laboratory and the optimum effective dosage in small field trials. These results should ensure the proper selection of the most appropriate formulation and dosage for the particular climatic and ecological conditions.