The purposes of the study were to assess and compare the validity of both 300-yd and 500-yd shallow water run (SWR) tests to predict peak aerobic power (VO2peak). Subjects included 18 women and 18 men who performed a graded exercise treadmill test to predict VO2peak and then performed a 300-yd and 500-yd SWR for time. In addition to SWR times, other independent variables included age, gender, body weight, height, leg length, percent body fat, and 300-yd and 500-yd SWR heart rate and rating of perceived exertion. Correlation coefficients with measured VO2peak were r = -.84 and -.87 for the 300-yd and 500-yd SWR times, respectively. Multiple regression analyses revealed that prediction of VO2peak from 300-yd SWR time improved by including gender and body weight (R = .919; SEE = 0.360 L.min-1). Similarly, prediction of VO2peak improved from 500-yd SWR time by including gender, body weight, and leg length (R = .940; SEE = 0.316 L.min-1). Equations were also developed for use in pools of varying water depths. In conclusion, the 300-yd and 500-yd SWR tests can provide accurate and valid estimates of aerobic power.