Classhouse and laboratory studies were conducted to investigate the effect of microorganisms on the activity of water-soluble phytotoxins from Brassica residues, and on the persistence of the phytotoxins. Warm temperatures (20–24°C) and long incubation periods (20 to 40 days) were conductive to reduced phytotoxicity while low temperatures (0–4°C, regardless of incubation time), or short incubation time (regardless of temperature) resulted in levels of phytotoxicity similar to that found without incubation. The removal of microbial populations by micro-filtration resulted in the maintenance of phytotoxicity, regardless of the incubation conditions. The quantity of residues or extracts from residues used in a pot experiment and the soil type (sand or clay soil) determined the degree of phytotoxicity. Generally, the greater the quantity of residues or extracts, the greater the toxicity, with residues being more toxic than extracts from the same rate of residue. These observations agree with the general literature that the level of toxicity is determined by the quantity of residue present, and the rate of decline in the toxicity of water-soluble toxins is dependent on the microbial populations present, and their level of activity.