A water-column apparatus is described in which settling rates of Bacillus thuringiensis serotype H-14 [B.t. (H-14)] formulations can be indirectly quantified using mortality of mosquito larvae at restricted depths as an index of B.t. (H-14) activity. To illustrate the type of data provided by this method, commercial B.t. (H-14) products (Bactimos, Teknar, Vectobac) and experimental formulations were compared at the manufacturers' recommended rates for mosquito control. All evaluations utilized laboratory-reared, 4th-instar Aedes aegypti larvae. The procedure can be used to provide an index of suspension properties of different B.t. (H-14) formulations and to measure dispersion rates of granular formulations resting at the bottom of a water column. Standardization of the method provides a convenient and practical means of generating comparative data on the effectiveness of B.t. (H-14) and other mosquito larvicides against specific target species.