Laboratory bioassays using Culex quinquefasciatus larvae evaluated the effect of tadpole shrimp on the persistence of Bacillus thuringiensis var. israelensis (B.t.i.) in water collected from the surface of field microcosms. Time elapsed since B.t.i. treatment, as well as presence or absence of soil and tadpole shrimp, affected B.t.i. persistence at the water surface in 15- and 30-cm total depths. The presence of tadpole shrimp slowed the natural decline in B.t.i. effectiveness over time, but this effect was depressed when soil was present. Tadpole shrimp foraged throughout the water column and stirred up the substrate, keeping more particles in suspension at the surface than in microcosms with no shrimp, in microcosms with water depths of 15 and 30 cm.