Eight series of chromosome substitution lines in the Chinese Spring background were subjected to natural freezing stresses during stem elongation. Two of the series were also subjected to an artificial freezing stress at stem elongation. In the series involving the winter cultivar Cheyenne, a major genetic component of the resistance to freezing injury during stem elongation was located in chromosome 5D. Among the seven other series screened in the field, each of the 21 substituted chromosomes was significant in at least one series. Chromosomes most frequently implicated were 3A, 6A, 2D, 4D and 5D. The genetic control of observed tiller mortality following a freezing stress was therefore considered genetically complex.