Forisomes are contractile protein bodies that control the effective diameter of the sieve elements of the faboid legumes by reversible, Ca2+-driven changes of shape. Forisomes consist of fibrils; we inferred from available electron-microscopical data (which necessarily provide images of fixed, non-functional forisomes) that a reversible assembly of ordered fibrillar arrays might be involved in the contractile mechanism. Here we examined functional forisomes isolated from Vicia faba L. by differential interference contrast microscopy and polarisation microscopy. We found them birefringent in the longitudinally expanded but not in the contracted state, showing 'parallel extinction' with the direction of vibration of the slow ray coinciding with their long axis (positive birefringence). These findings met predictions derived from the theory of form birefringence in rodlet composite bodies, and supported the idea of molecular self-assembly as a factor in forisome contractility.