To adhere to the principle of "exercise specificity" exercise testing should be completed using the same physical activity that is performed during exercise training. The present study was designed to assess whether aerobic step exercisers have a greater maximal oxygen consumption (max VO sub 2) when tested using an activity specific, maximal step exercise test (SET; arms and legs) versus a maximal running test (legs only). Female aerobic step exercisers (N=18; 20.7 plus or minus 1.5 years) performed three maximal graded exercise tests (GXTs): 2 SETs; 1 treadmill test (TMT). The SET consisted of six 3-min progressive stages of alternate lead, basic step, basic step with biceps curls, knee raise with pull-down, repeater knee with pull-down, lateral lunge with pull-down, and side squat with shoulder presses. Stepping rate was 32 steps- min super-1 on an 8-in (20.32 cm) step for stages 1-3, and a 10-in (25.4 cm) step for stages 4-6. Submaximal and maximal heart rate (HR) and oxygen consumption (VO sub 2) were recorded at the end of each stage. Test-retest reliability for the first five stages of the SET ranged from .91 to .97 for HR, and from .84 to .96 for VO sub 2. Maximal HR was significantly greater (p =.0001) for the SET (200 plus or minus 6.2 beats.min super -1) as compared to the TMT (193 plus or minus 7.9 beats -min super -1). No significant difference was found for max VO sub 2 (42.9 plus or minus 8.5, 41.2 plus or minus 5.9 ml-kg super -1.min super-1, p =.14). The SET was a valid and reliable protocol for assessing responses of these aerobic step exercisers; however, max VO sub 2 from a TMT did not differ significantly from the SET. Conversely, max HR obtained from the criterion TMT was 7 beats.min super -1 lower than from the SET. If a training HR for step exercise (arms and legs exercise) is prescribed based on the max HR from treadmill exercise (legs only), then the training HR should be calculated from a TMT max HR that has been increased by 7 beats.min super -1 to obtain an intensity of step exercise comparable to that of running. (Contains 6 tables and 4 figures.)