Degradation of the cotton defoliant thidiazuron and its photoproduct photothidiazuron by soil and thirteen species of microorganisms was examined. Aspergillus versicolor, Torula rosea, and Flavobacter sp. were most active in degrading thidiazuron. Unknown water-soluble metabolites and phenylurea were the major products. A. versicolor and Penicillium cyclopium were most active in degrading photothidiazuron. 4-Hydroxyphenylphotothidiazuron was the major organosoluble product formed by A. versicolor; phenylurea and an unidentified metabolite constituted the major organosoluble products from P. cyclopium. Both microbes also formed appreciable water-soluble metabolites. Radioactive carbon dioxide was formed from thidiazuron-aniline-14C by Oscillatoria sp. but not by Chlorella sp., suggesting that the former algal species utilized the defoliant as an energy source.