Recent experimental evidence does not coincide with the predictions of the different theories of ageing in Drosophila. A review of the evidence and more specifically of the experimental methods in use shows that in experiments one or more variables have in general not been taken into account. These variables are of different types; their influence on imaginal life span may be direct or indirect; direct through the action of con-temporaneous environmental factors, indirect through the action of parental or of antecedent environmental factors. An alternative theory of ageing is proposed; it essentially relates imaginal life span in Drosophila to the preimaginal rate of growth.