Acts as an anti-CsrA protein, binds CsrA and prevents it from repressing translation of its target genes, one of which is flagellin. Binds to flagellin and participates in the assembly of the flagellum. Overexpression leads to increased levels of FlaA and FlaB, but levels of FlaC remain stable (PubMed:27353476). Involved in post-transcriptional regulation of flagellin biosynthesis (PubMed:27353476). Interacts with flagellins FlaA and FlaB but not with FlaC; recognizes glycosylated and non-glycosylated FlaA equally (PubMed:27353476, PubMed:28659885). Interacts with CsrA (PubMed:27353476). May form a 3-way complex of flagellin, FliS and FliW simultaneously in which FliS and FliW do not directly interact (By similarity). Cells grow faster and are about 55% less motile in a swarming assay, flagellar filaments are severely shortened (PubMed:27353476). Decreased levels of flagellins FlaA and FlaB but not FlaC accumulate; 3-fold more flaA and 4-fold less flaB mRNA accumulate in mid-log phase (PubMed:27353476). In double csrA-fliW deletions, flagellin levels are slightly lower than wild-type (PubMed:27353476). Belongs to the FliW family.