A mutational approach was taken to identify genes required for low-temperature growth of the chilling-tolerant plant Arabidopsis thaliana. The screen identified mutants that were specifically compromised in their ability to grow at 5 degrees C but were indistinguishable from wild type when grown at 22 degrees C, The populations screened were mutated either by ethyl methanesulphonate or by T-DNA insertion, In both cases symptoms at 5 degrees C included chlorosis, reduced growth, necrosis and death, This diversity of phenotypes demonstrates roles for chilling-tolerance responses in such diverse processes as organelle biogenesis, cell metabolism and cell and organ development. Cosegregation analysis on the first five mutants isolated from the T-DNA lines indicated that in three of them, pfc1, pfc2 and sop1, the chilling phenotype is the result of T-DNA insertion in a gene required for chilling tolerance rather than the creation of a temperature-conditional mutation in an essential housekeeping gene, This identification of T-DNA tagged alleles will facilitate cloning of the PFC1, PFC2 and SOP1 loci and allow for the biochemical and molecular genetic characterization of these chilling-tolerance genes and the proteins that they encode