This study investigates the effects of square mesh barriers of various mesh size on escape success of the blue crab, Callinectes sapidus, under laboratory conditions. A total of 743 crabs was caught by beach seine and held in a shore-based aquarium, in circular tanks (circle divide 4 m x 0.9 m) with 0.3 m water depth. Using barriers constructed with square mesh of known size (35, 40, and 45 mm bar length), the proportion of crabs escaping through each mesh size was measured for each half centimetre carapace width (CW) class. A logistic model was fitted to the size frequency data of the retained and escaped blue crabs for each mesh size in the experiment. L-50 for the 35, 40, and 45 mm (bar length) square meshes tested in this study was 8.09 (s.e. 0.117), 9.32 (s.e. 0.088), and 10.56 (s.e. 0.114), respectively. This paper provides the first results for blue crab square mesh selectivity, which could be used as a reference point to indicate the most appropriate mesh sizes to be tested in crab traps during more comprehensive sea trials.