ANALYSIS of entrainment in circadian systems depends on a knowledge of the phase resetting effects of the light (or temperature) pulses perturbing the circadian oscillation. Thus when a light pulse of a particular intensity and/or duration falls at certain phases of the free-running oscillation it may generate phase-advances (+Δφ) in subsequent activity; when it falls at other phases it may generate phase-delays (−Δφ). A plot of such phase changes, both magnitude and sign, as a function of the phase of the oscillation so perturbed, is called a phase response curve (PRC) (refs 1, 2). In a systematic study of the resetting effects of light pulses on the rhythm of pupal eclosion in Drosophila pseudoobscura, Winfree3 demonstrated two topologically different types of PRC, depending on the strength of the signal. When light pulses were ‘weak’ ( 50 s) phase shifts became abruptly larger, up to 10–12 h. Because of the way Winfree's data were graphically presented, weak resetting curves were called type 1 and strong curves type 0. An abrupt change from type 1 to type 0 has been observed only in circadian systems which control physiological events such as pupal eclosion (refs 1–3 and D.S. in preparation) and oviposition5 in populations of insects. Rhythms of locomotor activity in single animals, on the other hand, have only shown low amplitude type 1 curves6,7. Entrainment in ‘physiological’ rhythms is in most cases mediated by extraoptic photoreceptors and a hormonal output8. ‘Behavioural’ rhythms, on the other hand, most frequently involve entrainment by way of the organised photoreceptors9,10 and an electrical (neural) output10–12. Truman used these differences to distinguish two types of biological clock13. Here, however, we report PRCs which switch from type 1 to type 0 in the locomotor activity rhythm of a cockroach Nauphoeta cinerea, which might from earlier literature have been expected to show only type 1 resetting.