The purpose of this investigation was to assess the validity of a 500-yard shallow water run test to determine peak aerobic power (VO2peak) and to compare it with the commonly used 1.5-mile run test. Subjects included 15 men and 28 women who completed two trials each of a 500-yard shallow water run test and a 1.5-mile run and then completed a graded exercise test on a treadmill to determine VO2peak. Correlation coefficients with measured VO2peak were -0.89 and -0.80 for the 1.5-mile and 500-yard water runs, respectively. Multiple regression analyses revealed that prediction of VO2peak from 1.5-mile run time was significantly improved by including the 1.5-mile run peak HR (R2 = 0.82, SEE = 3.52 ml.kg-1.min-1). Similarly, for the 500-yard water run test, percent body fat and height were significant additional predictors of VO2peak (R2 = 0.86, SEE = 3.19 ml.kg-1.min-1). We conclude that the 500-yard shallow water run test, especially when used with the descriptive measures of percent body fat and height, can provide a reasonable estimate of an individuals' cardiorespiratory fitness classification. However, caution is advised when administering these tests in older populations or in those with multiple coronary risk factors.