The objective of this investigation was to observe and evaluate the effects of surfactants on the biodegradation of toluene in the subsurface. Soil microcosms containing gasoline were used to simulate conditions of an aquifer contaminated with hydrocarbons. The surfactants were the nonionics, Surfonic N-40 and N-95. Three volumes of gasoline, 0.005, 0.05, and 0.1 mL, were injected into separate microcosms in order to observe changes in toluene concentration in both adsorbed component conditions and free-phase, interstitial gasoline conditions. The data presented are the results from the microcosms containing 0.005 mL gasoline. -Dissolved toluene was only partially biodegraded in the microcosms containing free-phase hydrocarbons. The lack of complete biodegradation was believed to be due to substrate toxicity.