The rarefaction method proposed by Sanders (1968) to allow one to compare the diversity of samples composed of different numbers of individuals and species is compared with the results that would be obtained if smaller random samples were taken from a universe defined by the observed sample. The rarefaction technique overestimates the number of species that would be expected by sampling. The magnitude of the overestimation is strongly influenced by the distribution of individuals among species in the original sample and by the presence of even moderate spatial aggregation. Four diversity indices, one of them newly proposed here, are compared in terms of their response to changes in the numbers of individuals and species, in the distribution of the individuals among the species, and in spatial aggregation. To increase comparability, it is suggested that scaled values be used; the scaling should be done in terms of the possible range of values of the index for the given numbers of individuals and species. T...