Ripples is a human -machine musical duet where the human performance stimulates a cascade of responses from gener a- tive music software. The work emphasizes chordal and a r- peggiated material derived, in the machine’s case, from u n- derlying patterns of oscillation. The Ripples software listens to MIDI input from the musician’s performance and r e- sponds in real -time with ge nerated MIDI output. It is d e- signed for live performance by two instruments but may be adapted for other configurations. The musician is expected to play in a style influenced by , and sympathetic to , the character of the generative output. The structure an d dur a- tion the performance is at the discretion of the musician. The beha vior of the generative software is fixed, but does adapt by ‘listening’ to the musician’s performance. This a r- ticle details the algorithmic processes of the generative sy s- tem, the typ es of interactions and adaptations employed, and provides descriptions and examples of some contrasting pe r- formances of the work. The design of Ripples demon strates how a relati vely simple generative process , co upled with nuanced interactive elements, can give rise to a musical metacreation system (a compositional design) that is ae s- thetically effective in impr ovised duet performances with a sympathetic human mus ician.