TERMINATION of diapause in the spider mite Tetranychus urticae is considered to depend solely on a period of chilling, the so-called cold rest1–3. The minimal period of chilling required to reactivate a certain percentage of the overwintering population may vary between populations of different geographic origin4. No unambiguous relationship, however, has been established between chilling period and latitude, for there may be appreciable differences in the minimal period of cold rest between strains from close localities5. The situation seemed more complex after Dubynina's6 discovery that reactivation of a Russian strain of this species increased linearly with time when mites had been induced to diapause under the influence of long days, abrupt temperature changes and severe competition for food, while reactivation was cyclical when diapause had been induced by short days and relatively low temperatures. In the latter case maximal percentages of reactivating females were obtained after diapausing mites had been kept in cold storage for 40 and 75 d. Minimal percentages were obtained after 50 and 95 d. Reactivation was complete by about the 105th day of storage. Experiments in England, however, produced no evidence for cyclical reactivation in British strains of T. urticae5. Using a Dutch strain of T. urticae, which has a constant photoperiodic reaction and has been reared in the laboratory for more than 15 yr (ref. 7), we have now found that termination of diapause in this species depends not only on a period of chilling but on photoperiod as well.