Of various benzoate-utilizing bacteria tested, Alcaligenes eutrophus 335, A. eutrophus H16, A. eutrophus JMP222, A. eutrophus JMP134, Alcaligenes strain A7, and Pseudomonas cepacia were able to grow with 4-fluorobenzoate as the sole source of carbon and energy. P. cepacia also utilizes 3-fluorobenzoate. Except for A. eutrophus JMP134, which is known to grow with 2,4-dichlorophenoxyacetate and 3-chlorobenzoate (R. H. Don and J. M. Pemberton, J. Bacteriol. 145:681-686, 1981), the strains were unable to grow at the expense of these compounds or 4-chlorobenzoate. Assays of cell extracts revealed that all strains express dienelactone hydrolase and maleylacetate reductase activities in addition to enzymes of the catechol branch of the 3-oxoadipate pathway when growing with 4-fluorobenzoate. Induction of dienelactone hydrolase and maleylacetate reductase apparently is not necessarily connected to synthesis of catechol 1,2-dioxygenase type II and chloromuconate cycloisomerase activities, which are indispensable for the degradation of chlorocatechols. Substrate specificities of the dienelactone hydrolases provisionally differentiate among three types of this activity. (i) Extracts of A. eutrophus 335, A. eutrophus H16, A. eutrophus JMP222, and Alcaligenes strain A7 convert trans-4-carboxymethylenebut-2-en-4-olide (trans-dienelactone) much faster than the cis-isomer (type I). (ii) The enzyme present in P. cepacia shows the opposite preference for the isomeric substrates (type II). (iii) Cell extracts of A. eutrophus JMP134, as well as purified dienelactone hydrolase from Pseudomonas strain B13 (E. Schmidt and H.-J. Knackmuss, Biochem. J. 192:339-347, 1980), hydrolyze both dienelactones at rates that are of the same order of magnitude (type III). This classification implies that A. eutrophus JMP134 possesses at least two different dienelactone hydrolases, one of type III encoded by the plasmid pJP4 and one of type I, which is also present in the cured strain JMP222.