In inbred Mus musculus several different beta chains are known. In certain strains two beta chains are produced in unequal amounts by the two closely linked genes of the doublet breeding unit allele Hbb(d): Betadmaj and betadmin. One strain has a variant doublet allele, Hbb(p), which produces a variant minor beta chain, betapmin (the major beta chain, betapmaj, may not differ from betadmaj chain). Certain other strains have a singlet allele, Hbb(s), that produces only one beta chain, betas. Other species have different beta-chain patterns. In M. cervicolor two variant major beta chains are found, betacmaj (d-like) and betacmaj (s-like), both of which were found associated with minor beta chains. M. caroli has only one, 'Leporelike' beta chain, with structural features characteristic predominantly of betadmin chain in the N-terminal half and of betadmaj chain in the C-terminal half. The present paper presents sequence data on betas, betadmaj, betadmin, betapmin and betacmaj (d-like) chains. The data on betadmin chain cover almost the whole of that chain and show a minimum of nine differences from betadmaj chain and two from betapmin chain. It is suggested that the data on the beta chains of the various species show evidence for the past occurrence of double crossovers over regions within a gene coding for only one or a few amino acids, which events can be explained by the 'hybrid DNA' models of genetic recombination. Supplementary information on the amino acid sequence of the proteins has been deposited as Supplementary Publication SUP 50067 (36 pages) at the British Library Lending Division, Boston Spa, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1976) 153, 5.