The rate of sex-linked recessive lethal mutation was determined in males of 2 strains of Drosophila melanogaster (Oregon-R and Canton-S) raised as larvae on medium containing caffeine, or injected as adults with caffeine solution. No influence of caffeine on mutation rate could be detected. The conclusion of Andrew (Am. Naturalist 93: 135-138, 1959) that caffeine acts as a mutagen in Drosophila is not supported. The disagreement may be due to physiological differences between the strains of flies employed, or to the occurrence of unrecognized mutant clusters in Andrew s results.