A new species overlap measure is defined as the probability that, give
n a randomly selected species is present in at least one of the two co
mmunities, it is present in both communities. This measure can be thou
ght of as a community analog of the Jaccard index for species overlap
between two samples. Standard maximum likelihood methods are used to e
stimate the parameters of a modified beta-binomial distribution. This
leads directly to estimates of the community overlap measure. In an ex
ample from decapod communities on coral reefs. it is shown that this n
ew estimate of community species overlap is 50% larger than the common
ly used Jaccard measure of species overlap.