This qualitative study investigated why women of low socio-economic status (SES) are less physically active than women of higher-SES. Semi-structured interviews were conducted with 19 high-, 19 mid- and 18 low-SES women. A social-ecological framework, taking into account intrapersonal, social and environmental level influences, was adopted to guide the development of interview questions and interpretation of data. Thematic analysis identified a number of key influences on physical activity that varied by SES. These included negative early life/ family physical activity experiences (a consistent theme among those of low-/mid-SES); participation in a wider range of physical activities in leisure time (high-SES); greater priority given to television viewing (low-SES); lack of time due to work commitments (low-SES); lack of time due to family commitments (high-SES); and neighbourhood-level barriers (low-SES). Financial costs were not perceived as a key barrier by women in any SES group. Public health strategies aimed at reducing SES inequalities in physical activity might focus on overcoming negative early experiences/attitudes to physical activity, reducing television viewing and promoting a wider variety of different types of physical activity, and addressing neighbourhood safety and other barriers to physically active lifestyles in socio-economically disadvantaged areas.