Abstract: The long-term impact of fertilizer regimes on soil microbial biomass carbon (SMBC), soil microbial biomass nitrogen (SMBN) and soil enzyme activities were studied over the growing season in a fluvo-aquic soil in Changping County, Beijing, China. Four different fertilization treatments were established in 1991. They were in a wheat-maize rotation receiving either no fertilizer (CK), mineral fertilizers (NPK), mineral fertilizers plus swine manure (NPKM) or mineral fertilizers with maize straw incorporated (NPKS). Results indicated that long-term fertilizer regimes had considerable effects on SMBC, SMBN and enzyme activities compared to CK. The order of SMBC and SMBN in all fertilization treatments was NPKM NPKSNPKCK, and they were significantly different among these treatments (P0.05). Studies on soil enzyme activities showed that soil urease, invertase, alkaline phosphatase activities were significantly enhanced in the treatments applied with fertilizers, and were much higher in the treatments applied of mineral fertilizers with organic manure or maize straw (NPKM, NPKS) than in NPK treatment. Catalase activity was markedly decreased in these treatments throughout the season from April to September. Further analysis indicated that treatment and sampling date had obvious influence on the SMBC, SMBN and soil enzyme activities separately and there were significant interactions between treatment and sampling date.