Drosophila melanogaster, which had been reared under standard conditions (25 degrees and a 24-hr light/dark cycle involving 12 hr of light) were exposed, on the first day of adult life, to four environments (all at 25 degrees ) as follows: (i) a 24-hr day consisting of 12 hr light and 12 hr dark; (ii) a 21-hr day (10.5 hr light, 10.5 hr dark); (iii) a 27-hr day (13.5 hr light, 13.5 hr dark); and (iv) constant light. The experiment was repeated four times. In all four experiments the flies on a 24-hr day lived significantly longer than the flies in the other environments. This result, comparable to other observations on plants, indicates that eukaryotic systems as oscillators perform most effectively when they are driven close to their natural "circadian" frequency.