To gain an insight into vertebrate genome evolution, we have analysed the organization of an approximately 40-kb genomic clone of an amphioxus (Branchiostoma floridae) cosmid library. Amphioxus is considered as being the last non-vertebrate relative to vertebrates. Sequencing and analysis of the above clone using three different exon prediction programs (GRAIL, GENSCAN, MZEF) have led to the identification of a gene of the aldo-keto reductase family as well as further exons that gave a significant database match to known genes. (C) 1999 Elsevier Science B.V. All rights reserved.