We present InstListener, a system that takes an expressive mono- phonic solo instrument performance by a human performer as the input and imitates its audio recordings by using an existing MIDI (Musical Instrument Digital Interface) synthesizer. It automatically analyzes the input and estimates, for each musical note, expressive performance parameters such as the timing, duration, discrete semitone-level pitch, amplitude, continuous pitch contour, and continuous amplitude contour. The system uses an iterative process to estimate and update those parameters by analyzing both the input and output of the system so that the output from the MIDI synthesizer can be similar enough to the input. Our evaluation results showed that the iterative parameter estimation improved the accuracy of imitating of the input performance and thus increased the naturalness and expressiveness of the output performance.