The researchers examined if scores from the original Psychological Need Satisfaction in Exercise Scale (Wilson, Rogers, Rodgers, & Wild, 2006) were invariant from a modified version specific to physical activity and then examined measurement invariance of scores across groups on the modified scale. Three groups were examined: (a) Students/staff from a university (N = 283), (b) a sample drawn from the general population (N = 214), and (c) individuals living with osteoporosis (N = 221). Measurement invariance was tested with four nested models using increased equality constraints per model. Results of invariance tests between two versions of the Psychological Need Satisfaction in Exercise Scale and between groups that completed the Psychological Need Satisfaction in Exercise Scale modified to physical activity supported configural and weak invariance of scores (i.e., equivalent factor structure and loadings). As such, the constructs of competence, autonomy, and relatedness were construed similarly across versions of the instrument and across two groups. Strong invariance (i.e., equivalent intercepts) was not supported, and therefore, direct score comparisons should be made with caution. (Contains 3 tables and 6 footnotes.)