It has been shown previously that when utrophin is highly expressed in mice which lack dystrophin, the muscle pathology is prevented. Immunohistochemical evidence strongly suggests that utrophin in these transgenic mice occupies the position normally filled by dystrophin, although it is not possible to establish this firmly at the level of the light microscope. Using the higher resolution provided by the electron microscope, we demonstrate here by immunogold labelling with both monoclonal and polyclonal antibodies that utrophin, in both its truncated and full-length forms, is indeed specifically located in the subcellular position usually occupied by dystrophin in normal muscle. Moreover, when double-labelling of utrophin and beta-dystroglycan was carried out, colocalisation of the two labels was often seen, indicating an association of the two proteins. Furthermore, when both utrophin and dystrophin were labelled in a transgenic line in which both were simultaneously expressed, the sites of both proteins were in the same zone in relation to the plasma membrane. When both proteins were present, the density of labelling of each was reduced compared with when they are expressed individually, suggesting that there is a finite number of binding sites. These results constitute further support for the view that utrophin might be therapeutically substituted for dystrophin in dystrophic muscle.