Listeners rated the subjective difference between pairs of different synthesized orchestral instrumental timbres. They then reported their perception of variations on crossing ascending and descending musical scales played in different timbres. With little or no timbre difference between the scales, auditory streaming by pitch led to the perception of separate high and low halfscales. Increasing timbre difference led to the perception of the complete scales played by each instrument, particularly if the two scales were in distant keys or temporally interleaved rather than simultaneous. If the notes of each scale alternated between two instruments, then perceptual separation by timbre would result in the perception of the "jumping" sequence of notes played by each instrument. This sequence was perceived only if the scales were discordant or temporally interleaved. Multidimensional scaling of the difference ratings led to three dimensions, which were correlated with acoustic parameters resulting from spectral analysis of the sounds. The most important aspects of timbre for perceptual separation were the proportion of energy in the lower partials and, for discordant scales, the duration of the decay. Auditory streaming by timbre thus depends on particular dimensions of timbre and on musical factors such as harmonic relation, simultaneity, and continuity.