The fine structure of leech body wall muscles, has been analysed under great length variations. All the measurable parameters of the fibre (distance between thick filaments, sarcomere and A band width, percentage of actin crowns around myosin filaments, fibre radius) at different elongations were tested with equations describing the geometrical model of helical fibres. Anaesthetized and nonanaesthetized worms behave in different ways and can be utilized to verify the 'changing partner hypothesis'. The fit between theoretical and experimental data suggests that a double change of partner actually takes place in extremely stretched fibres.