Enoyl-ACP reductase (ENR, also known as FabI) has received considerable interest as an anti-bacterial target due to its essentiality in fatty acid synthesis. All the Fabl structures reported to date, regardless of the organism, are composed of homo-tetramers, except for two structures: Bacillus cereus and Staphylococcus aureus FabI (bcFabl and saFabl, respectively), which have been reported as dimers. However, the reason for the existence of the dimeric form in these organisms and the biological meaning of dimeric and tetrameric forms of Fabl are ambiguous. Herein, we report the high-resolution crystal structure of a dimeric form of Bacillus anthracis FabI (baFabI) and the crystal structures of tetrameric forms of baFabI in the apo state and in complex with NAD(+) and with NAD(+)-triclosan, at 1.7 angstrom, 1.85 angstrom, 1.96 angstrom, and 1.95 angstrom, respectively. Interestingly, we found that baFabI with a His6-tag at its C-terminus exists as a dimer, whereas untagged-baFabI exists as a tetramer. The His6-tag may block the dimertetramer transition, since baFabI has relatively short-length amino acids ((255)LG(256)) after the 310-helix -q7 compared to those of FabI of other organisms. The dimeric form of baFabI is catalytically inactive, because the a-helix a5 occupies the NADH-binding site. During the process of dimer-tetramer transition, this a5 helix rotates about 55 toward the tetramer interface and the active site is established. Therefore, tetramerization of baFabI is required for cofactor binding and catalytic activity. 2017 Elsevier Inc. All rights reserved.