A mutation has been found that dramatically shortens the period of the circadian locomotor rhythm of golden hamsters. The pattern of inheritance of this mutation suggests that it occurred at a single, autosomal locus (tau). Wild-type animals have rhythms with free-running periods averaging about 24 hours; animals heterozygous for the mutation have periods of about 22 hours, whereas homozygous animals have rhythms with periods close to 20 hours. Animals that carry the mutant alleles exhibit abnormal entrainment to 24-hour light:dark cycles or are unable to entrain.