The aim of this study is to analyze a set of socializing agents for sport and physical activities and to establish their relationship with leisure time sport and physical activities behaviors and practice patterns in samples of teenage students with different sociocultural backgrounds. The sample included 2168 students in their first year of secondary education, 423 of them being from Costa Rica, 408 from Mexico, and 1337 from Spain (1052 male students, 1037 female students, and 79 students who did not specify gender) aged 11-16 years old ( M = 12.49; SD = .81). A validated questionnaire with questions about leisure time sport and physical activities and socializing agents was used. Descriptive, inferential, and multinomial logistic regression analyses were carried out with SPSS 17.0 to compare all three countries. Costa Rica had the most active students, best friends' inactivity, and unsupportive parents being the agents predicting inactivity and a low level of sport and physical activities. Mexico has a high dropout rate and inactive students exceed active ones; no agent predicts inactivity or sport and physical activities pattern. Spain has the highest level of sport and physical activities practice, and parents, siblings, and friends are predicting agents of inactivity together with unsupportive parents and friends.