We investigated the effects of listening to medium-to-fast tempo asynchronous music on performance, heart rate (HR), perceived exertion, and affect during an acute bout of swimming at a self-selected pace. Healthy college-aged recreational swimmers (n=20; M_age=20.3±2.0 years) were studied on two occasions in randomised order: swimming 1,200 m while listening to music (125-140 beats per minute) or during a no-music control trial. Following a period of habituation to the SwiMP3 audio player, HR, rating of perceived exertion, feeling scale responses and performance time trials were assessed for each 200 m during the 1,200 m freestyle swimming trial, each interspersed with a one min rest period. Participants swam significantly faster during the asynchronous music condition relative to control (P<0.01, η²_p=0.32). Although music had no significant influence on perceived exertion, the music condition was associated with more favourable arousal (P<0.01, η²_p=0.40) and affective (P<0.05, η²_p=0.19) responses. These findings suggest that both recreational and competitive swimmers may benefit from the use of underwater MP3 players and music.