Abstract. Bioremediation of crude oil in salt marsh mesocosms growing Spartina alterniflora was investigated during winter and summer to determine the influence of nitrogen (N) and phosphorus (P) fertilization, flooding, and season. Fertilization with urea and ammonium (NH +) applied at 75 or 150 kg N ha-i with or without P did not significantly (p = 0.05) increase oil or hydrocarbon degradation in continuously flooded mesocosms over an 82 day period during winter (temperature range of 17 to 30 ~ Phosphorus applied at 40 kg P ha-i significantly (p = 0.05) increased oil and hydrocarbon degradation. Nitrate (NO~) added alone did not increase oil or hydrocarbon degradation, but when added with P, it significantly (p = 0.05) increased degradation above that for P alone. Up to 70% of applied oil and 75% of applied hydrocarbons were degraded in P supplemented treatments. Inipol, an oleophilic fertilizer containing N, P, and a dispersant, significantly increased oil and hydrocarbon degradation. During a 40 day summer experiment (temperature range of 2742 ~ N and P fertilization did not increase oil or hydrocarbon degradation. For continuously flooded treatments, 72% of applied hydrocarbons were degraded while 51% were degraded in alternately flooded treatments. Mesocosms provided conditions suitable for quantitative recovery of oil and results indicated that N and P fertilization, flooding, and season interacted to influence oil bioremediation. Even under the most favorable conditions, more than 1 month was required for most of the oil to disappear.