Some of the relationships between a lobster (Homarus americanus) and the dimensions of its shelter have been examined. Field studies showed that lobsters generally occupied shelters in which the height was less than the width, and that there was a correlation between lobster size and shelter size. In the laboratory lobsters chose low profile shelters when given a choice of flat (height = ½ width) versus square (height = width) profile openings. Back doors were often present in the field, and laboratory experiments suggested that some sort of constriction before the rear opening was necessary to qualify it as a back door. The amount of time spent in eight opaque or transparent shelter under bright, dim, and dark illumination was measured. In bright and dim light with opaque shelter, lobsters spent significantly more time in a shelter than in the other treatments, showing darkness to be major factor in shelter selection. Shelters are excavated by pushing sand or gravel with the third maxillipeds in a bulldozing motion or by digging with the first two or three pairs of walking legs, followed by a vigorous fanning of the pleopods.