Capture-recapture data on common volesMicrotus arvalis (Pallas, 1779) in central Europe have been almost exclusively analysed by means of the enumeration technique (minimum number alive or calendar of catches). Here we compare enumeration and Jolly-Seber (JS) estimation of population size in the common vole using live-trapping data from an alfalfa field-population in southern Moravia, Czech Republic. Over the entire study the enumeration estimate of the population size was smaller by an average of 28% than the JS estimate. The negative bias increased with density, decreased with both capture probability and the survival rate, and was more pronounced in males at high density. We conclude that the method of direct enumeration is not reliable for estimating population size in the common vole.