As part of an investigation of the immune system in the developing zebrafish, Danio rerio, we cloned and characterized a genomic V(H) segment and a number of cDNAs encoding IgM heavy chains. The genomic V(H) has the characteristic features of V(H) in other vertebrates, including a leader segment interrupted by a short intron, and regulatory and recombination signal sequences. The V(H) sequences of 75 cDNA clones fell into four distinct groups or families. All of these families cluster most closely with other teleost V(H); one of the families, V(H)3, is also closely related to other vertebrate V(H). Analysis by Southern blotting suggests that there are a number of germline genes corresponding to each family. Most variability within the V(H)1 family is confined to codons 30-32, at the end of the first framework region and in the beginning of the first complementarity-determining region. The nucleotide sequence of the genomic V(H) in this region contains motifs associated with a relatively high frequency of somatic mutation. There is much variability in segments of the clones corresponding to the third complementarity-determining region. The cDNA sequences encoding the fourth framework region are consistent with the contribution of four distinct J(H) segments.