The fate of toxic organic compounds in aquatic environments depends, in part, on the capacity of microorganisms to metabolize these compounds at very low concn. Little information exists on the fate of aromatic ring compounds at low concn (<100 mg/l) in aquatic environments. Therefore, we attempt to characterize the degradation of the aromatic compound phenylacetic acid (PAA) at low concn by aquatic bacteria. Three bacterial isolates, PAL-1, PAL-10, and PAL-100 were obtained from continuous culture enrichment experiments at PAA concn of 1, 10, and 100 mg/liter, respectively. Washed cell suspensions of each isolate metabolized PAA, as measured by oxygen uptake, without lag regardless of whether cells were grown in acetate or PAA minimal salts medium, suggesting PAA metabolism was constitutive. Acetate-grown PAL-1 and PAL-100 cultures showed greater oxygen uptake activity in the presence of PAA than did PAA-grown cultures. All three isolates had high activity in the presence of meta-hydroxy-PAA. Incubation of washed cell suspensions at 25C for 12 h showed that only the PAL-1 isolate lost activity (75%) relative to cell suspensions incubated at 4C. The studies provide physiological criteria that may characterize the types of bacteria that transform aromatic compounds at low concn in aquatic environments.