Abstract The head part of the myosin heavy chain (MHC) represents the essential component of the molecular force-generating system of muscle [1–3]. To date, three fast but only one slow MHC isoforms have been identified in adult mammalian limb muscles [4, 5]. We show here two functionally different slow MHC isoforms, MHCIβ and MHCIa, coexisting in a considerable fraction of slow fibres of rabbit plantaris muscle. The two isoforms exhibit distinct electrophoretic mobilities and different kinetic properties. Thus, as it is known for the fast muscle, also the slow muscle seems to use different MHC isoforms in order to fulfil different functional demands.