The purpose of this study was to characterise two new mouse mutants, carousel, and whirligig. Both were derived from a large-scale mutagenesis programme which screened for dominantly inherited mutations that cause hearing impairments and balance defects. Genetic mapping placed both mutations on the proximal region of chromosome 4. Paint-filling and clearing techniques revealed abnormalities of the lateral semicircular canal. Scanning electron microscopy showed increased numbers of outer and inner hair cells in the apical region of the organ of Corti. The behavioural, genetic, and morphological characteristics lead us to the conclusion that both mutants are probably alleles of seven previously identified mutants which all map to proximal chromosome 4 and share similar defects of the lateral semicircular canal. We suggest that this region may be particularly susceptible to ENU mutagenesis independent of genetic background.