AbstractMicrocosms containing sediment and water from lacustrine, riverine and estuarine ecosystems were used to determine the rate of mineralization of t‐butylphenyl diphenyl phosphate (BPDP), a triaryl phosphate ester which is commonly used as a flame‐retardent plasticizer. Mineralization of BPDP ranged from 1.7% up to 37.2% after 8 weeks and the rate of degradation was related to the nutrient level and contaminant history of each ecosystem. Total heterotrophic populations as well as populations of microorganisms capable of utilizing BPDP as a sole source of carbon and energy varied but were greatest in sediments with higher BPDP mineralization rates. Phosphodi‐ and phosphotriesterase enzyme activities varied in each sediment according to BPDP mineralization rates. Adaptive increases in enzymatic activities and microbial populations were observed in some microcosms after exposure to BPDP. Volatile and nonvolatile organic‐solvent extractable 14C‐residues were quantitated by liquid scintillation spectrometry. Extractable 14C‐residues were separated by thin‐layer chromatography into polar and non‐polar compounds. Our results indicate that the rate of BPDP biodegradation varied among ecosystems and was highest in water and sediment from eutrophic ecosystems which have been previously exposed to xenobiotics. Therefore, microcosm components should be collected from ecosystems which have physical, chemical, and biological characteristics which are similar to the ecosystems most likely to be contaminated by a xenobiotic.