The drag coefficient (C d ) of grayling Thymallus thymallus was dependent on body surface conditions and rigidity. At comparable flow conditions, C d values of a fish preserved in formalin (high body rigidity) were 15-30% lower than those obtained on a freshly-killed fish (medium rigidity); the presence of skin mucus on fish could reduce C d by 10%. The hydrodynamic potential of grayling increased during ontogenesis, because C d values decreased (except for yolk sac larvae, which had a particular morphology) and the swimming capacities (in terms of relative muscular mass) increased. Grayling morphology evolves towards hydrodynamically efficient shape at high velocities, and there is a relationship between these shifts in hydrodynamic abilities and the different habitats (in terms of current velocity) used by five morphological groups. Therefore, the concept of hydrodynamic potential (i.e. hydrodynamics of shape and swimming capacities) could be a useful tool in fish ecomorphology and predictions of habitat use. 2000 The Fisheries Society of the British Isles