Dietary restriction (DR) extends life span in many organisms, through unknown mechanisms that may or may not be evolutionarily conserved. Because different laboratories use different diets and techniques for implementing DR, the outcomes may not be strictly comparable. This complicates intra- and interspecific comparisons of the mechanisms of DR and is therefore central to the use of model organisms to research this topic. Drosophila melanogaster is an important model for the study of DR, but the nutritional content of its diet is typically poorly defined. We have compared fly diets composed of different yeasts for their effect on life span and fecundity. We found that only one diet was appropriate for DR experiments, indicating that much of the published work on fly "DR" may have included adverse effects of food composition. We propose procedures to ensure that diets are suitable for the study of DR in Drosophila.