What is the best slogan to motivate older people to increase their levels of physical activity? This online survey of 1200 Australians aged 50 years and older tested the relative effectiveness of five different campaign slogans. It found that the slogan: ‘Use it or lose it‘ performed best on measures of internal and external motivation, perceived effectiveness, liking, believability, and personal relevance, suggesting it could be used to encourage older physical activity in older people.