ABSTRACT Protoanemonin is a toxic metabolite which may be formed during the degradation of some chloroaromatic compounds, such as polychlorinated biphenyls, by natural microbial consortia. We show here that protoanemonin can be transformed by dienelactone hydrolase of Pseudomonas sp. strain B13 to cis -acetylacrylate. Although similar K _m values were observed for cis -dienelactone and protoanemonin, the turnover rate of protoanemonin was only 1% that of cis -dienelactone. This indicates that at least this percentage of the enzyme is in the active state, even in the absence of activation. The trans -dienelactone hydrolase of Pseudomonas sp. strain RW10 did not detectably transform protoanemonin. Obviously, Pseudomonas sp. strain B13 possesses at least two mechanisms to avoid protoanemonin toxicity, namely a highly active chloromuconate cycloisomerase, which routes most of the 3-chloro- cis , cis -muconate to the cis -dienelactone, thereby largely preventing protoanemonin formation, and dienelactone hydrolase, which detoxifies any small amount of protoanemonin that might nevertheless be formed.