1. The wavelength, tail-beat frequency and trailing-edge amplitude have been measured for five groups of rainbow trout at various subfatigue cruising speeds. Four groups of fish were fitted with extra drag loads. The swimming mode was anguilliform by definition, but is probably best considered as intermediate between this and the carangiform mode.
2. The wavelength of the propulsive wave represented 0.76 of the body length. The specific amplitude (amplitude/length) tended to reach a maximum value of 0.175 at tail-beat frequencies approaching 5/sec.
3. The product of frequency and specific amplitude was found to be linearly related to swimming speed in all five groups of fish.
4. The critical swimming speed for the non-loaded control group was 1.73 body length/sec, and fell in groups 1-4 as the magnitude of the extra drag loads increased. The critical swimming speed for the control group is low for salmonids, probably as a result of the unfavourable history of the fish.
5. A method is described for calculating the drag of a swimming fish from the effects of the extra loads on the characteristics of the propulsive wave. It was found that thrust, T = 7.9 (swimming speed)1.79. The thrust was approximately 2.8 times greater than that required for an equivalent straight rigid vehicle.
6. It was calculated that the power output of the red muscle system would need to be about 0.48-0.77 ergs/sec/g muscle to overcome the drag of the fish at cruising speeds.
7. The power output of the fish was compared with values calculated by means of mathematical models proposed by Taylor and Lighthill. It was found that the fish did not fit the assumptions made in Taylor's model, and so power output calculations were not comparable with those calculated in the present paper. Lighthill's model was found to give values which were within 5 % of the values calculated here at higher swimming speeds. At lower swimming speeds the values were up to about 50 % lower than expected because again the fish did not fit the assumptions involved.
8. The relationship between thrust and swimming speed was extended into the sprint-speed range. It was calculated that fish could reach a maximum sprint speed maintained for 1 sec, provided that drag was reduced by about a half, or that the power required was that to accelerate the fish to that speed.