A column study was initiated to study the effect of phenanthrene, fluorene, and p-cresol on the aerobic biodegradation of carbazole in columns of sandy aquifer material. Biodegradation of the contaminant mixture was sequential in space with p-cresol being preferentially degraded, followed by phenanthrene, then the other compounds. Both p-cresol and phenanthrene were completely biotransformed to non-detectable levels during passage through the 46 cm sand column but some carbazole and fluorene persisted throughout the approximately 3 month experiments. Influent p-cresol (10000ppb) was the only compound that affected adaptation of the microbial community to carbazole biodegradation, but its effect was of little practical importance, amounting to a 4.5 day difference in carbazole breakthrough. However, when influent p-cresol was at high levels (70 000 ppb), biotransformation of the other co-substrates in the mixture never ensued because p-cresol caused complete dissolved oxygen depletion. Conversely, influent p-cresol ultimately enhanced biotransformation of the other co-substrates in the mixture when present at a concentration (10000ppb) that did not deplete all available oxygen. The concentrations of the other, more recalcitrant compounds, ranging between 33 and 238 ppb, were probably too low to support bacterial growth so that slow, limited biotransformation resulted, although addition of an auxiliary substrate (i.e. the p-cresol) stimulated their biotransformation. Under quasi-steady-state conditions, the presence of phenanthrene in the influent inhibited fluorene biotransformation and possibly carbazole biotransformation. Results of the present study demonstrated also that interactions identified in static batch microcosms and in a hydrodynamic saturated column system can differ.