We studied Cr fractionation in three semi-arid soils (cultivated, under-the-canopy, and outside-the-canopy soils). The soils were amended with: Cr3+, Cr6+, tannery sludge, Cr3+ tannery sludge, and Cr6+ tannery sludge and all soils were incubated for 30 and 120 days at 25 degrees C. The Cr in three semi-arid soils was fractionated using sequential extraction (Tessier scheme). Data of Cr fractionation were used to find the correlations with microbial activities determined in previous work. The microbial activities: CO2-C evolved, dehydrogenase activity and nitrification were determined in the same soils amended with the same treatments. Tannery sludge was added at 0.0125 g g(-1) soil and Cr3+ or Cr6+ at 250 mu g g(-1) soils. After 120 days of incubation, higher values of concentration of Cr were found in the residual fraction in the three soils amended with all the treatments, except cultivated and outside-the-canopy soils amended with Cr6+ tannery. The non-residual fraction tended to decrease with time except in cultivated and outside-the-canopy soils amended with Cr6+ tannery sludge. CO2-C evolved was significantly correlated (p < 0.05 and p < 0.001) with fractions bound to: Mn oxides, Fe oxides organic matter at 30 and 120 days of incubation (from r= 0.827 to 0.979). Dehydrogenase activity was correlated with fractions bound to Fe oxides and bound to organic matter, and nitrification with fraction bound to organic matter at 30 days of incubation (r=0.874, 0.959, and 0.803, respectively). These results suggest that even in a sparingly available Cr fraction in semi-arid soils has effect on microbial activities. (C) 2007 Published by Elsevier B.V.