We tested the effects of 9 d of a high-carbohydrate diet (80% of calories as CHO, 80% CHO diet) vs. a moderate-CHO diet (43% of calories as CHO, 43% CHO diet) on the abilities of collegiate swimmers to maintain a high intensity of interval swim training. Interval swim times and other physiological indices were recorded the last 5 d of each diet. Swim-interval distances ranged from 50-m interval sets to continuous 3000-m swims. There were no diet effects on mean swim velocities for any interval distance, and mean (+/- SEM) velocities for all swims were identical for both diets. There were no diet effects on the physiological indices; however, postswim blood lactate concentrations were higher after the 80% CHO diet. When mean +/- SEM daily caloric intake is 19.56 +/- 2.16 MJ (4675 +/- 516 kcal) for swimmers undertaking swim training to develop aerobic capacity, an 80% CHO diet provides no advantage over a 43% CHO diet for maintaining interval-swim-training intensity.