In Ireland, both wild and cultured Mytilus spp. are widely distributed on all coasts. Mytilus edulis, Mytilus galloprovinicialis and hybrids of both species are found on the north, west and south coast of Ireland while M. edulis is found on the east coast. In Wales, it is believed that M. edulis is exclusively present. This study investigated the health status of these mussels. Samples of Mytilus spp. were collected from twenty-four sites encompassing all coasts of Ireland and the Welsh coast at different times of the year over several years. In total, eight hundred and forty-one Mytilus spp. were examined. Habitat description and the environmental factors influencing the study sites were recorded. All Mytilus spp. samples were screened using histology to determine health status and the presence of potential pathogens. At certain study sites a polymerase chain reaction (PCR) was carried out to differentiate which mytilid species were being screened. M. edulis was the most common mytilid species observed and was present at all study sites on all coasts of Ireland and the sole species along the Welsh coast. M. galloprovincialis was found together with M. edulis extensively on the southern and western coasts of Ireland and hybrids were identified in those samples. Though hybrids were also detected at one site on the northern coast and two sites on the southern coast of Ireland; M. galloprovincialis was not present at those sites. The prevalence of infection and/ pathological changes varied among the sites and were observed in both mytillid species and hybrids.