Circadian clocks continue to oscillate in constant conditions with their own period (tau) and entrain to a cyclic environment by adjusting their intrinsic period to that of the zeitgeber. When circadian clocks are released from entrained to constant conditions, the tau of their initial free-run often depends on the nature of the prior zeitgeber. These postentrainment effects on period (tau-aftereffects) have predominantly been reported for animals but, so far, not fungi. The authors therefore investigated tau aftereffects in the classic circadian model system Neurospora crassa. The standard laboratory strain frq+, the short-period mutant frq(1), and the long-period mutant frq(7) were entrained to 11 different photoperiods in a 24-h day (2-22 h) and to zeitgebers with six different T (16-26 h), and then released to constant darkness. tau-Aftereffects in response to different photoperiods correlated weakly with prior photoperiod in frq+ and were unsystematic in both period mutant strains. Strength and direction of the tau-aftereffect in zeitgeber cycles with different T depended on their length and on the strain, showing a negative correlation with zeitgeber length in frq+ and positive correlations in frq(1) and frq(7). It has been proposed that tau-aftereffects are based on interactions of oscillators within a cellular network. The present findings in Neurospora, which grows as a syncytium, suggest that tau-aftereffects also exist in circadian systems based on multioscillatory networks organized at the molecular level.