In plants, low fluences of ultraviolet-B radiation (UV-B; 280-320 nm) trigger the activation of protection mechanisms and various photomorphogenic responses. We established a novel screen to isolate Arabidopsis T-DNA mutants (uli, UV-B light insensitive) with reduced sensitivity towards UV-B light-mediated inhibition of hypocotyl growth. One of the mutants, uli3, shows also a reduced sensitivity of UV-B-induced gene expression events and was, therefore, chosen for further investigation. The ULI3 gene encodes an 80-kDa protein with potential domains for heme- and diacylglycerol-binding. In transiently transfected protoplasts, the ULI3:GFP fusion protein is localised in the cytoplasm but also adjacent to membranes. In etiolated Arabidopsis seedlings irradiated by UV-B, ULI3 mRNA expression is strongly up-regulated. This is caused by elevated transcription as demonstrated using stable transformants where a GUS-reporter was driven by the ULI3-promoter. ULI3 is preferentially expressed in the outer cell layers in leaves, stems and flowers, but not in roots. There is evidence that ULI3 represents a specific component involved in UV-B-mediated signal transduction in higher plants.