Caveolin-1 was the first protein identified that colocalizes with the ≈10-nm filaments found on the inside surface of caveolae membranes. We have used a combination of electron microscopy (EM), circular dichroism, and analytical ultracentrifugation to determine the structure of the oligomers that form when the first 101 aa of caveolin-1 (Cav 1–101 ) are allowed to associate. We determined that amino acids 79–96 in this caveolin-1 fragment are arranged in an α-helix. Cav 1–101 oligomers are ≈11 nm in diameter and contain seven molecules of Cav 1–101 . These subunits, in turn, are able to assemble into 50 nm long × 11 nm diameter filaments that closely match the morphology of the filaments in the caveolae filamentous coat. We propose that the heptameric subunit forms in part through lateral interactions between the α-helices of the seven Cav 1–101 units. Caveolin-1, therefore, appears to be the structural molecule of the caveolae filamentous coat.