Spores of Bacillus thuringiensis subsp. israelensis and their toxic crystals are bioencapsulated in the protozoan Tetrahymena pyriformis, in which the toxin remains stable. Each T. pyriformis cell concentrates the spores and crystals in its food vacuoles, thus delivering them to mosquito larvae, which rapidly die. Vacuoles containing undigested material are later excreted from the cells. The fate of spores and toxin inside the food vacuoles was determined at various times after excretion by phase-contrast and electron microscopy as well as by viable-cell counting. Excreted food vacuoles gradually aggregated, and vegetative growth of B. thuringiensis subsp. israelensis was observed after 7 h as filaments that stemmed from the aggregates. The outgrown cells sporulated between 27 and 42 h. The spore multiplication values in this system are low compared to those obtained in carcasses of B. thuringiensis subsp. israelensis-killed larvae and pupae, but this bioencapsulation represents a new possible mode of B. thuringiensis subsp. israelensis recycling in nontarget organisms.