CERTAIN molluscan smooth muscles contain protein filaments with a regular and characteristic fine structure1–3 to which the name paramyosin has been given4. Neither the chemical nature nor the function of these filaments is understood, and attention is once more drawn to these problems by the recent discovery in the same muscles of an asymmetric globulin5,8 precipitating in the form of needle-shaped ‘crystals’. This was found6–8 to have the amino-acid pattern of a tropomyosin, and the large amounts present in slow lamellibranch adductor muscles suggested that it might be responsible for the paramyosin structure, an inference supported by some preliminary studies of the X-ray diffraction pattern5,7. These ‘crystals’ have now been studied by electron microscopy.