From the day of hatching, groups of about 150 blowflies Phormia terraenovae R.D. were continuously subjected to light-dark cycles (LD) of constant duration, ranging from 20 to 28 h (zeitgeber period T), or to constant illumination (LL). Survival was determined by the number of days after which only 10% of the flies in each group were still alive, and expressed as percent of the survival of a control group kept in T=24 h. With T equal to 25, 26 or 27 h, survival was 100%; it was reduced to about 90% in T=23 or 28 h, to 85% in T=22 h, and to 70% in T=20 h. In LL, survival was between 52% and 75%, depending on light intensity. From records of locomotor activity, obtained from a few flies in each group, it can be concluded that in all the LDconditions the activity rhythms were entrained by the zeitgeber. Several possible explanations of the reduction of survival in the non-24-h days and in LL are discussed.

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