Field experiments with a uniculm mutant of Proctor spring barley showed that abnormal ear development often resulted from late sowing. The ear abnormalities were twin floret, supernumerary parts, abnormal rachilla, opposite spikelets, one-ranked spikelets, branched rachis, collar -like and bare nodes, aborted apex, and tubular leaf. The degree of abnormality was unaffected by photoperiod. In both uniculm Proctor and uniculm Kindred, node number relative to normal genotype was decreased by high-temperature treatment and the proportion of abnormal rachis nodes and the severity of the abnormality were increased. The transfer of plants from high to low temperature and vice versa at different stages of development showed that high temperature had an almost immediate effect, inducing abnormal development, but that transfer to low temperatures allowed the resumption of more normal development. The abnormalities closely resemble those induced by 2, 4.D and it may be that the effect of the mutant gene is in some way to change the metabolism of auxin.