A methodology allowing the detailed assessment ofthe capacities of microflorae to degrade gasoline inaerobic conditions has been developed. It consisted inthe determination of the degradation of a gasolinemodel mixture in liquid cultures in optimalconditions. The gasoline model mixture contained 23representative hydrocarbons of gasoline (GM23). Thekinetics and extent of biodegradation were evaluatedby continuous overall monitoring of CO2production and final chromatographic analysis (usuallyafter about 30 days) of the consumption of eachhydrocarbon. The methodology was used with soil andwater samples from polluted and non polluted sites.The experimentation aimed at assessing thedistribution of the degradative capacities in theenvironment and the prospects for natural attenuationof gasoline. Nine microflorae were tested. The intrinsicbiodegradability (existence of mechanisms ofbiodegradation) appeared total for GM23 as shown bythe results obtained with several microflorae. Thedegradative capacities of microflorae from nonpolluted samples were high (total degradation rates atleast 85%). Incomplete degradation was observedessentially for trimethylalkanes(2,2,4-trimethylpentane and 2,3,4-trimethylpentane)and for cyclohexane. In several cases, samples frompolluted sites exhibited more extensive degradativecapacities, with total degradation of all hydrocarbonsbeing observed for three out of the six samples.