The present study provides the first quantitative developmental analysis of movement in the weaver (wv/wv) mutant mouse. This autosomal recessive mutation affects both striatal and cerebellar circuitries that are related to motor performance. We report data on post-swim grooming behavior in 14 mutant and 14 control animals on alternate postnatal Days 13-20. Mutant animals showed a greater number, but shorter duration, of grooming bouts across this developmental period. The mutant animals also used external support during grooming, expressed various forms of ataxia, performed a higher proportion of smaller forelimb strokes than did the control animals, and failed to complete as many full stereotypic grooming sequences. These differences between mutant and control animals followed distinctive developmental courses. Our data demonstrate that previous anatomical and physiological characterizations of the weaver mutation have overt motor correlates.