A full-length cDNA clone (Onmy-UA-C32) encoding a major histocompatibility complex (MHC) class I heavy chain was isolated from a rainbow trout thymus cDNA library. Onmy-UA-C32 alpha I and III extracellular domains were most similar to other salmonids (92 and 86% at the nucleotide and amino acid level) but interestingly the alpha II domain is closer to that of the carp (74 and 73%) and zebrafish (75 and 70%). In addition, Onmy-UA- C32 displays conservation of residues known to be essential for the function and structure of MHC class Ia molecules. Northern blot hybridization with alpha 2 or 2-3 domain probes of Onmy-UA-C32 detected high expression (2.6 kh) of this gene in the spleen, thymus, kidney, heart and intestine with lower levels being observed in the brain and liver. No tissues were found to be negative indicating a ubiquitous pattern of expression for Onmy-UA-C32. Onmy- UA-C32 may therefore represent a MHC class Ia gene in trout as well as providing new insights regarding the evolution of the MHC within teleost species.