The flavoenzymes dimethylglycine dehydrogenase (EC 184.108.40.206) and sarcosine dehydrogenase (EC 220.127.116.11) contain covalently bound FAD linked via the 8 alpha-position of the isoalloxazine ring to the imidazole N(3) of a histidine residue (Cook, R. J., Misono, K. S., and Wagner, C. (1984) J. Biol. Chem. 259, 12475-12480). The flavin-peptides from tryptic digests of these two enzymes have been isolated and sequenced. Automated sequence analysis showed that the flavin-peptide from dimethylglycine dehydrogenase contained 25 amino acid residues in the following sequence: Ser-Glu-Leu-Thr-Ala-Gly-Ser- Thr-Trp-His(flavin)-Ala-Ala-Gly-Leu-Thr-Thr-Tyr-Phe-His-Pro-Gly-Ile-A sn-Leu-Lys. The sequence determined for the flavin-peptide from sarcosine dehydrogenase contained 14 amino acid residues Leu-Thr-Ser-Gly-Thr-Thr-Trp-His(flavin)-Thr-Ala-Gly-Leu-Gly-Arg.