An autosomal recessive behavioral mutation designated juvenile ataxia (ja) was first isolated from F2 progeny of four blonde (bl) Peromyscus maniculatus bairdi siblings. Juvenile ataxic homozygotes exhibit an ataxic gait without an associated tremor from 15 days postpartum. This ataxia continues but does not increase noticeably in severity until between 35 and 43 days of age. Thereafter, a rapid and dramatic improvement in the behavior pattern is seen, so that juvenile ataxic deermice over 45 days of age are phenotypically indistinguishable from wild type or heterozygotes of the same age. Female juvenile ataxic homozygotes rarely breed, while male homozygotes are fully fertile. Linkage tests between juvenile ataxia and blonde suggest that these two mutations are not allelic and are not tightly linked. Testing between juvenile ataxia and a previously described neurological mutant, boggler (bg), indicates that complementation between these two mutations is incomplete, and suggests that boggler and juvenile ataxic may represent allelic mutations.