The paper discusses Tedford's (1976) hypothesis concerning a sister-group relationship of the otariids with the ursids and the phocids with the mustelids, based on a cladistic analysis. It is concluded that because Tedford has used at most branching points in his cladogram the opposite character states as synapomorphies as those used in the sister-group, his hypothesis is put forward in conflict with the premises of the cladistic method. It must therefore be rejected.