Arginine succinate lyase (ASL) deficiency is one of the most common cause of urea cycle defect and shows all characteristics of this disorders. This disease is presented with hyperammonemia, abnormally kinky hair and mental retardation. 6-years-old-girl was brought to our hospital because of skin eruption, weight loses, abdominal pain, having no appetite, polydipsia and pollakiuria. In physical examination, especially occipital balding, mental retardation, hepatomegaly, ataxia and articulation defect were found. She, who was diagnosed as ASL deficiency after the laboratory examinations, had normal blood ammonia levels. Finally in this study, we emphasize; ASL deficiency must be thought when a child has mental retardation and cerebellar ataxia, even if normal ammonia levels.