The fungicidal activity, mammalian toxicity, and methods of synthesis of phthalimidophosphonothionates, a novel type of organophosphoramide compound, have been investigated. The findings led to development of the compound diethyl phthalimidophosphonothionate as a new bioproduct. This diester protects a variety of crops from certain plant diseases, such as powdery mildew, apple scab, leaf spot, brown rot, and black spot.