. 2003. The effects of a crude oil spill on microbiological indices of soil biological quality. Can. J. Soil Sci. 83: 173-181. The effects of a crude oil spill and remediation on the biological quality of an agricultural soil were examined based on three microbiological indices: microbial biomass carbon (MBC), dehydrogenase activity (DHA) and microbial metabolic diversity (MMD). The soils in both contaminated and uncontaminated plots were clay loam Gleyed Rego Black Chernozems developed from lacustrine materials overlying till. The four remedial treatments consisted of: meadow bromegrass (Bromus biebersteinii. Rohman and Schult), alfalfa (Medicago sativa L. cv. 'Algonquin'), fallow with wheat straw incorporation (SF) and unamended fallow (UF SP ). An unamended fallow on adjacent uncontaminated land served as a control (UF CON ). The spill had a statistically significant, negative effect on MBC and MMD (P = 0.01 and 0.05, respectively). Although a negative effect on DHA was not significant (P = 0.05), the results show evident trends. The forage treatments were, overall, significantly more effective than the SF treatment in restoring soil biological quality in hydrocarbon contaminated soil to control levels. Based on overall trends only, bromegrass had the greatest effect on soil biological quality relative to the UF SP , followed by alfalfa. The incorporation of wheat straw did not significantly affect any of the examined microbial indices. . Même si les effets négatifs de la pollution sur la DHA n'é-taient pas significatifs (P = 0,05), certaines tendances se dégagent des résultats. Dans l'ensemble, la culture d'espèces fourragères restaure la qualité biologique des sols contaminés par les hydrocarbures à celle de la parcelle témoin sensiblement mieux que le traitement SF. Si l'on ne se fie qu'aux tendances générales, le brome améliore le plus la qualité biologique du sol comparativement à la UF SP , suivi de la luzerne. L'incorporation de paille de blé au sol n'améliore pas significativement les indices de la qualité microbiologique du sol examinés.