The aspect ratio of the caudal fin of 63 fish species was measured. Swimming speeds for these species, representing 129 cases, were obtained from the literature for various fish sizes and speeds, i.e. "minimum speed" for sustained swimming and "maximum speed" (+ "burst" speed). Results show a very significant relationship of these two speeds with aspect ratio. The relationship described here allows for reliable estimations of individual speeds of the various species included. It also serves to illustrate the functional dependence of speed on the aspect ratio and length of the fish.