Two short pieces of freely improvised music by the same performer were recorded in microstructural detail by the use of a specially constructed automatic transcription apparatus. The apparatus consists of a modified DX7 synthesizer and 2650 microprocessor which interfaces with other computers for data processing. The resultant music is transcribed into a modified form of traditional notation and subjected to both micro- and macrostructural analysis. Microanalysis includes the areas of timing (interonset and duration distributions, displacement, chordal spreads, etc.), dynamics ( key velocity, quantization, chordal patterns, etc.), and legatoness (relative, absolute, pedaling). Macroanalysis uses the full panoply of devices from traditional music theory (tonal procedures, rhythmic and motivic design, pitch class sets, etc.). Correlations between microstructural parameters, and with macrostructure, were found to be highly significant in Improvisation A, which had a supplied external pulse, but largely absent in Improvisation B, which had no such pulse. Where pulse was present, rhythmic design was found to be based largely on pulse subdivision and shifting. Some performance effects (e.g., chordal spreads) operated over a time scale of 10 msec or less. Others (e.g., synchronization to an external pulse) showed less resolution. Differences in the distribution patterns of interonset times, durations, and legatoness suggest three independent underlying temporal mechanisms that may sometimes link together in coordination with macrostructure. Quantization ("categorical production") of some variables (interonset times, key velocities) was clearly established. The results were also interpreted in relation to an earlier model of improvisation (Pressing, 1987).