Single crystals of iron subjected to uniaxial tensile stress deformed by slip at temperatures down to about -145 $^\circ$C independently of their orientation. Between -145 and -196 $^\circ$C their behaviour depended on orientation and at -196 $^\circ$C crystals stressed in a direction within about 25$^\circ$ of a  direction fractured by cleavage without any prior deformation by slip. The range of orientation over which brittle cleavage occurred was increased at lower temperatures and all crystals tested were brittle at liquid-helium temperature. At any one temperature the brittle orientation range was increased by additions of small amounts of nitrogen or carbon to the iron, by ageing the crystals at room temperature, and by comparatively short exposure to neutron irradiation. The results of the tests were consistent with the cleavage strength obeying a critical normal stress law. Some evidence is given which suggests that fracture may be initiated at the intersection of a pair of twins in the appropriate (001) plane.