According to ecological models of behavior, the physical environment can influence physical activity engagement by a series of mediating and moderating processes. This study tested such a model to identify factors relevant to older adults' engagement in moderate-vigorous physical activity. Sociodemographic, psychological, physical, and environmental factors were assessed in 432 Western Australians aged 60 and older. Moderate-vigorous physical activity was measured objectively using accelerometers. No environmental variables were related to engagement in moderate-vigorous physical activity either directly or indirectly. However, various individual-level factors were significant, indicating that these may be more important than environmental factors in locations such as Australia that have generally conducive environments and ambient conditions.