Aquaporins (AQPs) accelerate the movement of water and other solutes across biological membranes, yet the molecular mechanisms of each AQP's transport function and the diverse physiological roles played by AQP family members are still being defined. We therefore have characterized an AQP in a model organism, Drosophila melanogaster, which is amenable to genetic manipulation and developmental analysis. To study the mechanism of Drosophila Malpighian tubule (MT)-facilitated water transport, we identified seven putative AQPs in the Drosophila genome and found that one of these, previously named DRIP, has the greatest sequence similarity to those vertebrate AQPs that exhibit the highest rates of water transport. In situ mRNA analyses showed that DRIP is expressed in both embryonic and adult MTs, as well as in other tissues in which fluid transport is essential. In addition, the pattern of DRIP expression was dynamic. To define DRIP-mediated water transport, the protein was expressed in Xenopus oocytes and in yeast secretory vesicles, and we found that significantly elevated rates of water transport correlated with DRIP expression. Moreover, the activation energy required for water transport in DRIP-expressing secretory vesicles was 4.9 kcal/mol. This low value is characteristic of AQP-mediated water transport, whereas the value in control vesicles was 16.4 kcal/mol. In contrast, glycerol, urea, ammonia, and proton transport were unaffected by DRIP expression, suggesting that DRIP is a highly selective water-specific channel. This result is consistent with the homology between DRIP and mammalian water-specific AQPs. Together, these data establish Drosophila as a new model system with which to investigate AQP function.