In the filamentous fungus Neurospora crassa the biosynthesis of carotenoids is regulated by blue light. Here we report the characterization of the albino-3 (al-3) gene of N. crassa, which encodes the carotenoid biosynthetic enzyme geranylgeranyl-pyrophosphate synthetase. This is the first geranylgeranyl-pyrophosphate synthetase gene isolated. Nucleotide sequence comparison of al-3 genomic and cDNA clones revealed that the al-3 gene is not interrupted by introns. Transcription of the al-3 gene has been examined in dark-grown and light-induced mycelia. The analysis revealed that the al-3 gene is not expressed in the dark and that its transcription is induced by blue light (Nelson, M. A., Morelli, G., Carattoli, A., Romano, N., and Macino, G. (1989) Mol. Cell. Biol. 9, 1271-1276). The al-3 gene encodes a polypeptide of 428 amino acids. Comparison of the deduced amino acid sequence of al-3 with the sequences of prenyltransferases of other species, from bacteria to humans, showed three highly conserved homologous regions. These homologous regions may be involved in the formation of the catalytic site of the prenyltransferases.