We found a new inner ear mutant exhibiting abnormal behavior, such as circling and head shaking, in a breeding stock of SJL/J mice. The traits are inherited in a simple autosomal-recessive fashion. Animals homozygous for the responsible gene, designated cosa, show no startle response to sounds and an inability to swim. In the inner ears of cosa/cosa homozygous, but not +/cosa heterozygous adults, histopathological features of severe damage that are typical for `cochleo-saccular' or `spotting' mutants have been demonstrated. We suggest here that the abnormal mice carry a mutation of a gene that is developmentally switched on in the early stages of development and is involved in endolymph homeostasis.