The whole-body concentration of the amino acid taurine was found to be more than 1000% higher during the adult stage of Drosophila melanogaster than during the larval stage. Drosophila larvae were killed by adding taurine (0.01 to 0.10 M) to their food medium. Adult Drosophila failed to produce progeny when fed 0.2 M taurine for one week. Lifetime feeding of taurine (0.05 to 0.20 M) produced no change in life span. Feeding the taurine precursor, hypotaurine, and the taurine mobilizing agent, beta-alanine, to Drosophila did not change life span at low concentrations but both decreased life span at higher concentrations. Taurine concentration in male C57BL/6J mice increased with aging in the heart, decreased in leg muscle and remained unchanged in brain, liver, kidney, and blood. We suggest that an as yet undefined developmental process is altered in Drosophila by taurine and that this process may be unique to insects.