Samples of human cricopharyngeus muscles obtained at postmortem were assessed for fibre type composition and fibre size. Fibre type was determined using serial cryostat sections exposed to several histochemical reactions; narrow fibre diameter and fibre area were measured from photomicrographs using a digitiser interfaced to a microcomputer. Results were compared with values from samples of vastus lateralis obtained from the same subjects. Cricopharyngeus muscle, in comparison with vastus lateralis, contained significantly more oxidative fibres but fewer glycolytic fibres and significantly more Type I fibres but fewer Type IIB. Cricopharyngeal fibres were significantly smaller than the fibres in vastus lateralis and in neither muscle were fibre sizes normally distributed. In each muscle most Type I fibres were oxidative, and the ratio of oxidative: glycolytic fibres was similar for Type IIA and IIB fibres. The fibre type proportions and fibre sizes in cricopharyngeus did not vary significantly with age or between males and females. The composition of cricopharyngeus--mostly Type I oxidative fibres and few Type II glycolytic fibres--correlated well with the functions of sustained tonicity to prevent aerophagia and occasional forceful contraction during deglutition.