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Soil enzyme activities (beta-glucosidase, amylase, chitinase, xylanase) were investigated in natural and improved fallows of the semi-arid zone of Senegal. The effect of age (4-, 11-, and 21-year-old), management (fenced versus grazed), and vegetation (natural, Acacia holocericea, Andropogon gayanus) were compared. Principal component analysis revealed a relationship between enzyme activities and the age and the management of fallows. beta-Glucosidase and amylase activities were significantly higher in the oldest natural fallows. The highest xylanase activity was recorded for the A. gayanus improved fallows. This fallow also showed highest chitinase activity, similar to that of the 21-year-old natural fenced fallow. Amongst the different types of fallow management studied, the introduction of A. holocericea depleted all the tested activities. No relationships between enzymes activities and soil organic content, and total microbial biomass were evident. The reasons for the observed variations are discussed. (C) 2001 Published by Elsevier Science B.V.