Bacillus thuringiensis serovar japonensis strain Buibui has the potential to be an important control agent for pest scarabs. Bioassays were designed to test B. t. japonensis against two of the major turf and ornamental scarab pests infesting turfgrasses and ornamentals and to serve as a basis for further tests against other scarab pests. LC and LD₅₀ values of B. t. serovar japonensis strain Buibui toxin and spores were determined by four different bioassays for the oriental beetle, Anomala orientalis (Waterhouse), and northern masked chafer, Cyclocephala borealis Arrow. Oriental beetle larvae were bioassayed in autoclaved and nonautoclaved soil from where they were collected (Kingston, RI [native]), in nonautoclaved soil from where the northern masked chafer larvae were collected (Groton, CT [foreign]), and per os. Northern masked chafer larvae were bioassayed in autoclaved and nonautoclaved soil from where they were collected (Groton, CT [native]), in nonautoclaved soil from where the oriental beetle larvae were collected (Kingston, RI [foreign]) and per os. LC₅₀ values of 3.93 µg toxin/g autoclaved native soil, 1.80 µg toxin/g nonautoclaved native soil, and 0.42 µg toxin/g nonautoclaved foreign soil and an LD₅₀ value of 0.41 µg per os were determined at 14 d for A. orientalis. LC₅₀ values of 588.28 µg toxin/g autoclaved native soil, 155.10 µg toxin/g nonautoclaved native soil, 265.32 µg toxin/g nonautoclaved foreign soil, and LD₅₀ of 5.21 µg per os were determined at 14 d (soils) and 10 d (per os) for C. borealis. There were significant differences in LC₅₀ values for oriental beetles in autoclaved, nonautoclaved native soil and nonautoclaved foreign soil. There were significant differences in LC₅₀ values for northern masked chafers in autoclaved and nonautoclaved native soil. B. t. japonensis can be applied now for control of oriental beetles at rates that are economically competitive with synthetic chemicals. If we can determine the component of nonautoclaved soil that enhances the activity of toxin, it may be possible to lower the rates of toxin needed for control to more economical levels for more difficult to control species such as the northern masked chafer.