We characterize the timing jitter of passively mode-locked, femtosecond, erbium fiber lasers with unprecedented resolution, enabling the observation of quantum-origin timing jitter up to the Nyquist frequency. For a pair of nearly identical 79.4 MHz dispersion-managed lasers with an output pulse energy of 450 pJ, the high-frequency jitter was found to be 2.6 fs [10 kHz, 39.7 MHz]. The results agree well with theoretical noise models over more than three decades, extending to the Nyquist frequency. It is also found that unexpected noise may occur if care is not taken in optimizing the mode-locked state. (C) 2010 Optical Society of America