The amino-terminal region of muscle tropomyosin is highly conserved among muscle and 284-residue non-muscle tropomyosins. Analysis of fusion and nonfusion striated alpha-tropomyosins and a mutant in which residues 1-9 have been deleted has shown that the amino terminus is crucial for function. The presence of 80 amino acids of a nonstructural influenza virus protein (NS1) on the amino terminus of tropomyosin allows magnesium-independent binding of tropomyosin to actin. The fusion tropomyosin inhibits the actomyosin S1 ATPase at all myosin S1 concentrations tested, indicating that the presence of the fusion peptide prevents myosin S1 from switching the actin filament from the inhibited to the potentiated state. Nonfusion tropomyosin, an unacetylated form, has no effect on the actomyosin S1 ATPase, though it regulates normally with troponin. Deletion of residues 1-9, which are believed to overlap with the carboxyl-terminal end of tropomyosin in the thin filament, results in loss of tropomyosin function. The mutant is unable to bind to actin, in the presence and absence of troponin, and it has no regulatory function. The removal of the first 9 residues of tropomyosin is much more deleterious than removal of the last 11 by carboxypeptidase digestion. We suggest that the structure of the amino-terminal region and acetylation of the initial methionine are crucial for tropomyosin function.