Bacillus sphaericus spores were suspended in bottles of filtered (0.45 microns) freshwater and seawater under various conditions of temperature, pH and salinity. Heat resistant culturable counts (spores) slowly decreased with time. Spores suspended in dialysis bags submerged in a freshwater pond or in flowing seawater underwent a more rapid drop in heat resistant spore counts than did spores held in bottles. Thus, laboratory studies may overestimate spore longevity in the environment. Spore settling rate was related to the nature of particulate material in the water column. Paraspores (or perhaps spores and toxin) of B. thuringiensis serovar israelensis (B.t.i.) had a greater tendency to adhere to and settle with suspended sediment and fine particulates than did paraspores of B. sphaericus. These observations may at least partially explain the greater persistence of B. sphaericus larvicidal activity in field tests than that of B.t.i..