Objective: The purpose of the current study was to gain a deeper understanding of the reasons that adolescent girls give 1) for ceasing participation in sports and other physical activities and 2) for why they do not participate as much as boys. Methods: 6 focus groups were conducted with adolescent girls (n = 49) aged between 13 and 15 years old. The focus groups were conducted at two metropolitan, co-educational public high schools in Adelaide, South Australia. Results: The girls generated a number of different reasons for ceasing to play sport, including losing interest, lack of competence and insufficient time. Girls also reported feeling like they were crossing traditional gender boundaries when playing sport, particularly for sports traditionally classified as 'masculine'. Additional concerns related to team-mates and teasing as well as concerns about appearance and image while playing sport. Conclusions: The findings identify a number of gender-specific reasons for girls' declining rates of participation in sport and physical activity and therefore suggest a number of strategies for improving girls' participation rates.