For decades, wood scientists and preservative formulators have employed the monocultured soil bottle assay to test efficacy of wood treatment in the laboratory as a rapid predictor of field performance. This study examines the effects of bicultured soil bottle assays on the decay by common wood decay fungi. Mycelial interactions were noted in early stages of colonization. With only two exceptions, a single fungus was apparent in each soil bottle, indicating dominance. The dominant fungi were not always the most efficient wood rots, and the rot type, white or brown, did not affect the dominance outcome on the preferred wood type.