Researchers attempting to show that music has positive effects on children need to understand and control for preexisting differences between those who do and do not select into musical participation in the first place. Within a large-scale, communitywide, prospective, longitudinal study of predominantly low-income, ethnically diverse students (N = 31,332), we examined characteristics of students who did and did not enroll in music elective courses (band, choir, orchestra, guitar, other) in public middle schools (sixth, seventh, and eighth grades) in Miami. Predictor variables included gender, ethnicity, poverty, special education, English language learner status, fifth-grade English proficiency, prior academic performance (fifth-grade grade point average [GPA], standardized math and reading test scores), and initial school readiness skills (social, behavioral, cognitive, language, and motor skills) at age 4. Only 23% of middle school students enrolled in a music class in sixth, seventh, or eighth grade, with band having the highest enrollment, followed by choir, orchestra, and guitar. Being male and having greater cognitive skills at age 4 and higher fifth-grade GPA and reading skills were related to later music participation. Black students, students in special education, and those not proficient in English were less likely to participate in middle school music classes. Results varied somewhat by type of music.