This study examined the efficacy of motivational interviewing (MI) for increasing physical activity in aging adults. Eighty-six participants aged 55 years and older were randomly assigned to receive either four weekly sessions of telephone-based MI for increasing physical activity, or a healthy activity living guide (information only control). Changes from baseline weekly caloric expenditure from physical activity, self-efficacy for physical activity, and stage of change for physical activity were compared across groups at posttreatment and six months follow-up. Results indicated that MI participants had higher weekly caloric expenditures from physical activity at posttreatment, but not at six months follow-up; higher self-efficacy for physical activity at six months follow-up; and demonstrated greater stage of change progression across assessments. These findings support the use of telephone-based MI for increasing physical activity in older adults in the short-term. Future studies will need to determine if follow-up booster sessions increase long-term efficacy.