The life span of adult Drosophila melanogaster fruit flies changed when they were fed two different photosensitizers. Methylene blue decreased the median life span by 49% when present in the food at a concentration of 0.001 M. Another photosensitizer, riboflavin, produced no changes in life span under the same conditions of a 12:12 h light/dark cycle at a daytime light intensity of 1000 lux. Flies exposed to constant darkness lived 43.2% longer than those exposed to constant light at a light intensity of 2000 lux. Under these conditions, riboflavin increased the life span of the flies exposed to constant light by as much as 25%. We conclude that riboflavin confers some degree of protection against the effects of constant light exposure. The completely different results obtained with riboflavin and methylene blue suggest a possible mechanism for photoageing involving photodynamic action mediated through the production of singlet oxygen.