This report is a contribution to the Strategic Environmental Assessment (SEA6) conducted by the Department of Trade and Industry (now Department of Energy and Climate Change). This report is an assessment of the status of the beds of the Horse Mussel Modiolus modiolus on the tide swept sea floor off north and northwest Anglesey, based on three short cruises of the RV Prince Madog during January - March 2005. In some places Modiolus forms dense beds that can carpet the sea floor and build up as reef-like features or bioherms. Side-scan sonar, sampling with grabs or dredges, and sledge mounted cameras were used to investigate the seabed. Modiolus was found, in sufficient abundance to be considered as 'beds', at four localities within the area surveyed. Modiolus beds are of considerable conservation interest, coming within the 'reef' category of the EU Habitats Directive, and they are listed under OSPAR. Modiolus beds are known to be highly vulnerable to physical disturbance; once destroyed, beds do not seem to recover naturally in the medium term. In addition to describing the occurrences of Modiolus and analysing the specimens sampled, the epifauna associated with them and other benthos in their vicinity are described.