Sensitivity to fundamental frequency (F0) differences was measured for two complex tones, A and B, which had the same F0 but were filtered into two different frequency regions. Tones were presented either alone or together. A signal-detection analysis was used to predict effects of combining F0 information across frequency regions. For 400-ms tones containing only unresolved harmonics, the first experiment showed that performance (in terms of d(')) for the combined presentation was better than for the isolated tones but was not optimal (assuming independent channels and noises) and was independent of the relative timing of pulses in the envelopes of tones A and B (varied by changing the starting phase of components of tone B relative to those of tone A). The nonoptimal performance was shown not to be due to peripheral masking (experiment II), or to listeners paying attention mainly to one frequency region (experiment III), nor was it specific to conditions where all harmonics were unresolved (experiment IV). In contrast, optimal performance in F0 discrimination for combined presentation was observed for 50-ms tones (experiment V). The results may reflect the limited ability of the human auditory system to integrate information simultaneously in the time and the frequency domains.