1. The fine localization of anti-troponin antibody along the thin filament and in the structure of troponin-tropomyosin paracrystal was studied by electron microscopy. 2. Anti-troponin antibody is distributed along the whole thin filament at regular intervals of 380 Å. 3. Anti-troponin antibody makes reconstituted troponin-tropomyosin-actin filaments form a bundle with transverse striations repeating with a periodicity of 380 Å. 4. The arrangement of tropomyosin molecules in the fibrous paracrystalline structure (Cohen-Longley type) was determined, mainly by examining the end portion of the paracrystals. 5. Taking the arrangement of tropomyosin molecules mentioned above and the previous finding that troponin binds to a localized region in tropomyosin paracrystals (Nonomura et al., 1968) into consideration, it was concluded that troponin binds to a region of the filamentous tropomyosin molecule 270 Å (two-thirds the molecular length of tropomyosin) from one end. 6. Tropomyosin treated with dextran sulfate derivatives forms a paracrystalline structure different from the Cohen-Longley type in the presence of divalent cations. The paracrystal shows 540 Å periodicity.