This dataset contains information on the abundance & biomass of infauna found in sediment grab samples carried out in Belfast Lough as part of a mussel (Mytilus edulis) bed stock assessment and Crepidula (Crepidula fornicata) survey in September 2012. The main aim of the survey was to assess the stock of mussel beds in Belfast Lough and to survey the Crepidula population in Belfast Lough. The infaunal surveying was carried out using a 0.1m2 day grab and a 1mm sieve. Samples were taken at 50 stations over 5 days in September in 2012- abundance & biomass were recorded and infauna were identified to species level where possible. Particle size analysis & Carbon and Nitrogen analysis was also carried out at each station. The most abundant species found and most frequently recorded was the polychaete Melinna palmata.