A field experiment was conducted to investigate the effect of six long-term (34-year) fertilizer and farmyard manure (FYM) treatments (Control, N, NP, NPK, NPK + S, NPK + FYM) and three physiological stages of wheat growth on the microbial biomass carbon (MBC), nitrogen (MBN) and dehydrogenase, mineralizable N and phosphatase activities in soil. It was found that a balanced application of NPK + FYM gave the highest values for the measured parameters and lowest at the control. Values were generally highest at tillering, followed by the flowering and dough stages. A significant positive interaction between fertilizer treatments and physiological stages of wheat growth was observed, being highest at maximum tillering due to application of NPK + FYM. Stepwise regressions have revealed that grain yield of wheat was significantly associated with mineralizable N at tillering (R-2 = 0.80), MBC at flowering (R-2 = 0.90) and alkaline phosphatase activity (R-2 = 0.70) at dough stages of wheat growth. (C) 2006 Elsevier Ltd. All rights reserved.