North-to-south clinal variation in morphology is described in Arthroleptis affinis of the Eastern Arc mountains of Tanzania. While northern populations are readily separable from other Arthroleptis species, southern populations are not. This complicates differential diagnosis. Despite this, it is concluded that in the present state of knowledge, it is more useful to include the clinal variation under the name A. affinis, rather than attempt to divide the species into groups that defy clear morphological definition.