The bacterial flagellum is assembled from over 20 structural components, and flagellar gene regulation is morphogenetically coupled to the assembly state by control of the anti-sigma factor FlgM. In the Gram-negative bacterium Salmonella enterica, FlgM inhibits late-class flagellar gene expression until the hook-basal body structural intermediate is completed and FlgM is inhibited by secretion from the cytoplasm. Here we demonstrate that FlgM is also secreted in the Gram-positive bacterium Bacillus subtilis and is degraded extracellularly by the proteases Epr and WprA. We further demonstrate that, like in S. enterica, the structural genes required for the flagellar hook-basal body are required for robust activation of σD-dependent gene expression and efficient secretion of FlgM. Finally, we determine that FlgM secretion is strongly enhanced by, but does not strictly require, hook-basal body completion and instead demands a minimal subset of flagellar proteins that includes the FliF/FliG basal body proteins, the flagellar type III export apparatus components FliO, FliP, FliQ, FliR, FlhA, and FlhB, and the substrate specificity switch regulator FliK.