Two novel immunoglobulin heavy chain (Igh mu) transcripts encoding membrane-bound forms of IgM (mIgM) were discovered in bighead catfish, Clarias macrocephalus. The first transcript contains four constant and two transmembrane domains [C mu 1-C mu 2-C mu 3-C mu 4-TM1-TM2] that have never been reported in teleosts, and the second transcript is an unusual mIgM that has never been identified in any vertebrate [C mu 1-(C delta 2-C delta 3-C delta 4-C delta 5)-C mu 2-C mu 3-TM1-TM2]. Fluorescence in situ hybridization (FISH) in bighead catfish, North African catfish (C. gariepinus) and hybrid catfish revealed a single copy of Igh mu in individual parent catfish, while two gene copies were found in diploid hybrid catfish. Intensive sequence analysis demonstrated multiple distinct structural variabilities in the VH domain in Clarias, and hybrid catfish were defined and used to generate diversity with various mechanisms. Expression analysis of Igh mu in Aeromonas hydrophila infection of the head kidney, peripheral blood leukocytes and spleen revealed significantly higher levels in North African catfish and hybrid catfish than in bighead catfish.