Native troponin-tropomyosin complex was isolated from Lethocerus indicus indirect flight muscle and tested for function. It was shown by rotary shadowing and by forming paracrystals on monolayers that the regulatory complex consists of a troponin head region approximately 130 Angstrom in diameter and a 400-Angstrom-long troponin T-tropomyosin tail. The complex forms paracrystals at the air-water interface on a positively charged monolayer. The globular head packs in rows 380 Angstrom apart which are bridged by the tail domain. Filamentous paracrystals were obtained by adding Mg2+ ions to the troponin-tropomyosin sample. These showed globular domains arranged in a regular pattern along ''ribbon''-like filaments. The spacing of the repeat was determined to be 380 Angstrom. (C) 1997 Academic Press.