Morphological and genetic evidence is presented which supports the existence of periostracal adventitious hairs on spat of the mussel Mytilus edulis. This character appears not to have been reported previously for Mytilus, and was thought to be restricted to a closely-related genus, Modiolus. The species identity of hairy mussel spat was confirmed by PCR (polymerase chain reaction) amplification of a diagnostic portion of the nuclear ribosomal DNA repeat unit (i.e. the ITS-2 region). Size-frequency analysis of spat, sampled in mid-September 1993, from rock pools and from the byssus of a nearby adult mussel bed, showed that hairy spat (mean shell length 1.87 mm, SE 0.17) were significantly (t=7.74; P<0·001) smaller than smooth-shelled spat (mean shell length 2.77 mm, SE 0.28), although not all small-sized individuals displayed this character. These findings suggest that there is a gradual loss of hairs (through abrasion or by ‘programmed’ loss) as the animal grows. We suggest that this character has some adaptive significance since it probably reduces predation by boring gastropods (e.g. juvenile Nucella lapillus) and may inhibit fouling, particularly by conspecifics, during the primary settlement phase.