The purpose of this study was to use the constructs of the theory of planned behavior (TPB) to gain a better understanding of the factors influencing older adults’ participation in strength training. Two hundred men and women age 55 years and older were purposely sampled from seniors’ centers in Ontario Canada. Participants completed a TPB questionnaire and reported their current physical activity participation. It was hypothesized that perceived behavioral control followed by attitude would be the strongest determinants of strength-training intentions and that intention would be the strongest determinant of strength-training behavior. Regression analyses revealed that subjective norm and perceived behavioral control explained 42% of the variance in intention and intention explained 40% of the variance in behavior. Gender and current strength-training participation did not significantly moderate the relationship between the TPB variables. The results suggest that interventions targeting subjective norm and perceived control might be helpful in promoting strength-training behavior among older adults.