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We model theoretically the response of the widely studied circadian oscillator of Neurospora crassa to inactivation of the frq gene. The resulting organism has been termed ‘‘arrhythmic’’ under constant conditions. Under entrainment to periodic temperature cycles Roenneberg, Merrow and coworkers have shown that the phase angle at which spore formation occurs depends on the entrainment period, curiously even in the null frq mutants ( frq 9 and frq 10 ). We show that such a response does not imply the presence of a selfsustained free-running oscillator. We derive a simple candidate model (a damped harmonic oscillator) for the null frq mutants that successfully reproduces the observed phase angle response. An endogenous period of 21 h for the damped harmonic oscillator coincides with the endogenous period of wild-type Neurospora. This suggests that the (noise driven) ‘‘residual system’’ present in the mutants may have a significant timekeeping role in the wild-type organism. Our model (with no change of parameters) was then used to investigate spore formation patterns under constant conditions and reproduces the corresponding experimental data of Aronson et al. (Proc. Natl. Acad. Sci. USA 91 (1994) 7683.) r 2004 Elsevier Ltd. All rights reserved.