Self-determination theory (SDT) postulates that environments providing psychological needs satisfaction (PNS) promote quality motivation, thus enhancing engagement and performance in diverse performance domains. SDT and PNS were used to investigate student experience of conservatoire instrument practice. Participants (N = 162) completed standardized self-report questionnaires. Associations between study variables were described and hierarchical multiple regression analysis was conducted to assess the predictive effects of autonomy and competence needs satisfaction and autonomous and controlled motivation on inter-subject flow variations. A considerable part of flow variations was accounted for by study variables. Results showed that conservatoire students, especially those studying instrument performance (n = 109), reported high levels of flow, perceived competence and autonomy, and intrinsic motivation. Autonomous motivation had a direct effect on flow variations, whereas controlled motivation had an inverse effect. Perceived competence was the strongest predictor of flow variations and it was also associated with external regulation (especially among students studying instrument performance), thus indicating that intrinsic and extrinsic motives both play important roles in conservatoire instrument practice. Conservatoire instrument teachers need to acknowledge that perceived competence is critical to flow in instrument practice, in order to provide their students with optimal challenges that avoid frustration and amotivation.