A new recessive mutant jimpy, jp, is described. It is situated on the differential segment of the sex chromosome and is lethal in the hemizygous condition. The jimpy mice show a behaviour disorder distinguished by an intention tremor first visible at about 11 days; after about three weeks they are liable to go into a convulsion if disturbed; death occurs usually between twenty-five and thirty-two days. jp shows about 21% recombination with Ta and 36% with Bn. It has not been tested directly against Br and Mo, but they both have been shown to be about four units beyond Ta with respect to Bn (Falconer 1954). The presumed order is therefore Bn, Ta, Br and Mo, jp.