Histological, histochemical and morphometric methods were used to investigate the cricopharyngeus muscle in the guinea-pig and to compare it with the extensor digitorum longus and soleus muscles. The cricopharyngeus comprised uniformly small diameter fibres otherwise similar in appearance to those found in skeletal limb muscles. Several fibre type profiles were distinguished within the cricopharyngeus, all of which had homogeneously high oxidative activity, whilst the majority were histochemically fast (Type II). Muscle spindles were not observed in the cricopharyngeus muscles. Compared to the surrounding musculature the cricopharyngeus has a higher oxidative activity and may thus be suitably adapted for the maintenance of tonic contraction, forming a part of the upper oesophageal sphincter.