Research has indicated that core training may be important because opposite rotational movements of the upper and lower torso are required in nearly all physical activities (Allers, 1981). More importantly, strong abdominal muscles assist in stabilizing the spine and can enhance in activities of daily life (Vera-Garcia, 2000). Abdominal muscles may be strengthened while exercising on a stable (e.g. floor) or unstable (e.g. stability ball) surface. There are equivocal results in the literature as to what surface elicits greater muscle activation (Hildenbrand & Noble, 2004; Piering, 1993; Sarti, 1996; Willet, 2001). Therefore, the purpose of this study was to compare EMG activity of the rectus abdominis and external oblique muscles during a crunch on a stable and an unstable surface. A two by two (site by surface) repeated measures ANOVA was used for this study. Data on the subjects (N = 12) indicated no significant differences between site during each condition (F(1,11) = 0.07, p > .05). Significant data were found between surface types (F(1,11) = 8.01, p < .05). The data suggest that the EMG values of the rectus abdominis and external oblique muscles were similar to each other on each surface. However, the floor condition elicited higher EMG values when compared to the stability ball condition. This suggests that the stability ball does not induce greater muscle activity when performing a crunch. Keyword(s): exercise/fitness, measurement/evaluation, research