Most adults with diabetes find physical activity difficult, and motivation can predict whether individuals will adhere to physical activity recommendations for self-managing their diabetes. Self-efficacy and autonomous motivation were studied as predictors of physical activity behaviour in adults with Type 1 diabetes or Type 2 diabetes, and consideration was given to participants’ stage of change for performing physical activity.  106 participants, recruited through various recruitment strategies from May to December 2010, completed questionnaires: Self-efficacy in Physical Activity Measure, Behavioral Regulation in Exercise Questionnaire, Stages of Change Measure, and modified Godin Leisure-Time Exercise Questionnaire.  Self-efficacy (beta = .23, p < .05) and autonomous motivation (beta = .28, p < .01) contributed to predicting physical activity behaviour, with autonomous motivation making the greater contribution.  Self-efficacy (r = .98) and autonomous motivation (r = .64) predicted positioning in the stages of initiating and maintaining regular physical activity, and autonomous motivation (r = .77) predicted positioning in the stage of performing irregular physical activity. Results suggest adults with diabetes may be more likely to perform physical activity if they are interested in and enjoy their physical activities, they are highly volitional in performing physical activity, and they also feel confident in their ability to overcome physical activity barriers.  It is recommended that diabetes care interventions to promote physical activity behaviour in adults with diabetes enhance self-efficacy and autonomous motivation, while considering dynamics of an individual’s stage of change.  Building interest and pleasure in physical activity and engaging inherent reasons for physical activity could assist individuals to initiate increased physical activity behaviour.  Once individuals are regularly performing physical activity, enhancing their beliefs in their ability to overcome physical activity barriers while keeping up their interest and enjoyment in physical activity could assist them to sustain their physical activity level over time.