The taxonomic status of three forms of the harvestman Mitopus morio in Britain have been examined. These have been identified by previous workers as a small, upland M, morio cinerascens, a larger, lowland M. morio morio, and a northern M. morio borealis. In this paper, the delimitation of trinomials has been found to be inappropriate for M. morio. since all three forms have been shown to lie at different points on a continuum of phenotypic variation within the species M. morio. It is suggested that this phenotypic variation is mediated by the variation in temperature regimes between different localities.