Staggerer is a neurological mutation of mice that affects the development of the central nervous system and causes abnormal behaviors. The staggerer cerebellum is already abnormal at birth and as the animal grows up there is a progressive loss of granule cells which have all disappeared by day 28. The earliest behavioral disturbance observed is a motor deficiency which occurs between 10 and 15 days-i.e. several days later than the appearance of the cortical abnormalities. To show that staggerer mutant mice also differ from normal mice in behavioral aspects before the age of 10 days, 28 staggerer pups and 246 normal pups aged from 1 to 9 days underwent different motor tests. In addition, the number of ultrasounds emitted during 40 s was recorded, and the animals were weighted every day. Differences between staggerer and normal mice were found as early as 3 days: staggerers were less efficient in motor tasks and they weighed less than normal mice. Staggerers also differed from normal mice in ultrasound production.