Littorina littorea (L.) were collected from a number of shores around Strangford Lough in Northern Ireland between 2004 and 2006. The length of each shell (cm) was measured with callipers. The basic unit of survey was a vertical transect, approximately 30 m wide, covering the vertical range of L. littorea on the shore. A surveyed site may contain up to two separate transects. Snails were collected using 0.25 m2 quadrats thrown haphazardly on the shore. These were stratified in three horizontal bands, with 8 quadrats in each band. On some shores the lower shore was muddy and no littorinids were found. In these cases, data were only collected from the areas where L. littorea were found and fewer quadrats were thrown. Length data are given for each transect in the file 'Shell lengths by transect'. Information on the associated shore, subsite code, year and number of quadrats thrown are in the 'Transect details' file.