COMPETITION theory predicts that intraspecific and interspecific competition should often have opposite effects on the use of resources by a population, the former increasing, the latter decreasing, the range of resource actually used1,2. Field data supporting these predictions are well known for the interspecific case3,4 but are scarce for the intraspecific condition, and we have been unable to find any reference demonstrating both effects within a single species. We therefore report here the verification of both predictions in respect of competition for space by the epiphytic bryozoan Alcyonidium hirsutum; less extensive data suggesting the same effects within other bryozoans are also reported.