To evaluate the genetic control of stress responses in Arabidopsis, we have analyzed a mutant (uvh6-1) that exhibits increased sensitivity to UV light, a yellow-green leaf coloration, and mild growth defects. We have mapped the uvh6-1 locus to chromosome I and have identified a candidate gene, AtXPD, within the corresponding region. This gene shows sequence similarity to the human (Homo sapiens) XPD and yeast (Saccharomyces cerevisiae) RAD3 genes required for nucleotide excision repair. We propose that UVH6 is equivalent to AtXPD because uvh6-1 mutants carry a mutation in a conserved residue of AtXPD and because transformation of uvh6-1 mutants with wild-type AtXPD DNA suppresses both UV sensitivity and other defective phenotypes. Furthermore, the UVH6/AtXPD protein appears to play a role in repair of UV photoproducts because the uvh6-1 mutant exhibits a moderate defect in the excision of UV photoproducts. This defect is also suppressed by transformation with UVH6/AtXPD DNA. We have further identified a T-DNA insertion in the UVH6/AtXPD gene (uvh6-2). Plants carrying homozygous insertions were not detected in analyses of progeny from plants heterozygous for the insertion. Thus, homozygous insertions appear to be lethal. We conclude that the UVH6/AtXPD gene is required for UV resistance and is an essential gene in Arabidopsis.