The gusmps/gusmps mouse is a model of the human lysosomal storage disease mucopolysaccharidosis type VII due to deficient beta-glucuronidase activity. We now report behavioural abnormalities associated with this single gene defect. In grooming, a developmentally regulated and genetically based activity, the mutant mice spent 1-5% of the normal time for body grooming and about 60% of the normal time in face grooming when stimulated with a light water mist. In the Morris water maze which tests spatial learning, the mutants could learn to locate an invisible platform but were deficient in remembering its location the next day or developing strategies to locate it in new positions. Thus, the gusmps/gusmps mouse demonstrates behavioural, memory and cognitive deficiencies suitable for monitoring functional restorations in therapy.