In 1999 divers discovered a population of the burrowing mantis shrimp Rissoides (Meiosquilla) desmaresti (Crustacea: Stomatopoda) east of the St Tudwal's Islands, North Wales. This species has only sporadically been recorded in UK waters and commonly occurs in the Mediterranean. In summer 2000 the burrow morphology and distribution of these shrimps in the area east of the St Tudwal's Islands was investigated. Burrows were found at ten of the 15 sites investigated in a survey area measuring ∼18 km2. Burrow density varied from one to 11 burrows per 100 m2. The burrows were always recorded in sediments consisting of a mixture of mud, sand and gravel but were not present at sites with a high proportion of mud (>70%) or sandy sediments with very little mud ([les ]2%). Resin casts of six burrows revealed that these have a simple elongated U-shape, with an average length of ∼450 mm and depth of ∼160 mm. The average diameter of the burrow entrance was 19±2 mm and the diameter of the burrow along the horizontal section varied between 18 and 38 mm with a distinct constriction part way along.