A measure of beta diversity is proposed for equal sized presence absence vegetation plots. The method begins by computing the minimum variance unbiased estimator of the expected species richnesE[Sn] as a function of sample size n for a set of N plots. Using semilogaritmic relation, beta-diversity is then determined from the slope of the best fitting line of the E[Sn] vs. log n plot. Beta-diversity is thus interpretable as the linear relation between expected species richness and log samples size. The method illustrated using 15 square plots of 100m2 each from a Matorral community in the rid Chaco of central Argentina. Further, some analogies between the proposed method and Wittaker’s multiplicative model of beta diversity are discussed.