As part of our research aiming to understand the genetic basis of the variation in morphological traits in ringed seals, we studied the muscles of mastication and their related skull characters of the Saimaa ringed seal (Phoca hispida saimensis). Six by-caught (by gill nets) Saimaa ringed seals in relatively good external condition were dissected for this study. The musculus temporalis muscle contributed more than half to the total jaw adductor muscle mass (mean 55.1 %). The masticatory muscle anatomy of the Saimaa ringed seal was similar to that reported for other fresh water seals but the m. digastricus of Saimaa ringed seal was large enough (25.2 percent of the total masticatory muscle mass) to cover the entire bulla. The skull of the Saimaa ringed seal is apparently not under significant cranial strain from any mastication muscles because they prey on small-sized fish and do not use their teeth (but claws) for scraping ice to form breathing holes. The apparently relatively high contribution of mm. pterygoidii of the Saimaa ringed seal to the jaw adductor muscle mass (14.5%) compared to canids (8.2 %) warrants further studies.