The flavoprotein component (SiR‐FP) of the sulfite reductase of E. coli is an octamer of the 66 kDa α subunit. It was shown to be cleaved in two peptide fragments. The 23 kDa fragment has been purified as a polymer of 8–10 subunits. It corresponds to the N‐terminal part of the native protein and was shown to contain essentially FMN as cofactor. The 43 kDa fragment is monomeric. It contains exclusively FAD and remains able to catalyze efficiently NADPH‐dependent reductions. One can conclude that each α‐chain of SiR‐FP is composed of two distinct domains, one binding FAD and the other FMN and that the FMN‐binding domains cooperate for a head‐to‐head subunit interaction.