A search was made for the genes responsible for the production of helical macrofibers in the original collection of macrofiber-producing strains of B. subtilis. Two loci were identified: fibA, located between hisA and tag-1, and fibB, linked to cysB. fibA governs a short-lived division suppression phenomenon associated with the production of rudimentary fibers, whereas fibB appears to be responsible for a persistent division suppression and a more highly organized helical macrofiber. Both mutations are recovered from each of the original macrofiber-producing strains which also carried the div IV-B1 mutation responsible for minicell production. The latter mutation by itself is not sufficient, however, for the production of macrofibers. Other known mutations leading to division suppression that map in the same region are shown not to be allelic to fibA or fibB. Neither fib locus appears to be responsible for helix hand determination.