We describe a 48-year-old woman with chronic constipation since early childhood who has an intestinal myopathy associated with a hitherto undescribed absence by immunostaining of smooth muscle alpha-actin confined to the intestinal circular muscle. There were no abnormalities in other contractile proteins (myosin, tropomyosin, filamin, caldesmon or desmin) and despite the abnormality of a contractile protein isoform in the circular muscle, no significant morphological changes were identified by light microscopy or ultrastructural examination. A possible developmental mechanism for the observed change is proposed. The use of specific antibodies to isoforms of contractile proteins may have potential value in the study of intestinal myopathies.