Three experiments tested the hypothesis that fundamental frequency (f 0) discrimination depends on the resolvability of harmonics within a tone complex. Fundamental frequency difference limens (f0 DLs) were measured for random-phase harmonic complexes with eight f0's between 75 and 400 Hz, bandpass filtered between 1.5 and 3.5 kHz, and presented at 12.5-dB component average sensation level in threshold equalizing noise with levels of 10, 40, and 65 dB SPL per equivalent rectangular auditory filter bandwidth. With increasing level, the transition from large (poor) to small (good) f0 DLs shifted to a higher f0. This shift corresponded to a decrease in harmonic resolvability, as estimated in the same listeners with excitation patterns derived from measures of auditory filter shape and with a more direct measure that involved hearing out individual harmonics. The results are consistent with the idea that resolved harmonics are necessary for good f0 discrimination. Additionally, f0 DLs for high f0's increased with stimulus level in the same way as pure-tone frequency DLs, suggesting that for this frequency range, the frequencies of harmonics are more poorly encoded at higher levels, even when harmonics are well resolved.