A recent study in our lab had suggested that aerobic oxidation of vinyl chloride (VC) was responsible for VC loss at extremely low oxygen concentrations that would be practically classified as anaerobic conditions. However, no aerobic VC-assimilating bacterium had been isolated from the enrichment cultures involved, and therefore no further characterization had been conducted. This study investigated the VC-utilizing kinetics of a Mycobacterium isolate from one of the enrichment cultures (from Cecil Field, FL), and another highly-enriched culture (from Industrial Site 4, CA). By limiting VC and O2 concentrations, thus separately limiting the utilization rates of respective substrates, various kinetic parameters of both cultures were determined. The maximum VC utilization rate (kVC) and half velocity constant for VC (Ks(VC)) of the Mycobacterium isolate were 11.5 nmol/min/mg of protein and 0.16 [MICRO SIGN]M, respectively, while those of the S4 culture were 4.7 nmol/min/mg of protein and 0.51 [MICRO SIGN] M, respectively. The half velocity constants for O2 (Ks(O2)) of the Mycobacterium isolate and S4 culture were 0.06 mg/L and 0.03 mg/L, respectively. There was no measureable O2 threshold identified for either culture. Overall, this study further supported the hypothesis that aerobic VC oxidation could account for loss of VC under commonly defined -anaerobic[DOUBLE VERTICAL LINE] conditions, and provided data for kinetic behavior of related aerobic VC-assimilating bacteria.