During post-embryonic development in animals, organs and tissues grow at different rates relative to each other, which is likely tied to the unique requirements of each organ’s function during development. Tissue-specific growth mechanisms can generate allometric differences that vary between individuals within a species and play critical roles in interspecific divergence. The fact that organs grow at different rates suggests that there must be tissue-specific mechanisms to control their differential post-embryonic growth trajectories, although surprisingly we know little about them. To gain a better understanding of these tissue specific growth mechanisms, we have been characterizing mutations that specifically alter the growth of the larval trachea in Drosophila melanogaster. Larval trachea growth is well suited for these studies since the trachea shows allometric growth during the larval stages, the trachea can be imaged and measured in living animals, gene expression can be specifically altered in the trachea using breathless-GAL4, and genes have been identified that appear to regulate tissue-specific growth in the larval trachea. Specifically, animals with mutations in uninflatable (uif) and Matrix metalloproteinase 1 (Mmp1) have larval tracheae that are roughly half the relative size of those in wild type animals. Through a screen of EMS-induced larval lethal mutations, we identified three allelic mutations that show substantially increased trachea to body length proportions in late third instar larvae. We named this gene rio based upon its long and winding tracheal phenotype. Whole genome sequencing followed by deficiency and specific mutation complementation tests revealed that rio mutations map to CG11340, which encodes a glycine gated chloride channel. Characterizing of rio mutants indicated that the enhanced tracheal growth occurs primarily in late 3rd instar, and in an extended late larval period (beyond when their heterozygous siblings pupariate). Enhanced tracheal size is not associated with increased cell number per metamere. In addition, the taenidia in rio mutant trachea appear normal. Interestingly, Uif expression appears to be generally higher in rio mutant trachea, with small numbers of individual cells showing a stochastic strong upregulation of Uif expression. RNAi of CG11340 in trachea using breathless-GAL4 recapitulates these tracheal phenotypes, whereas RNAi of CG11340 in various imaginal discs have no obvious effect on growth or differentiation of these tissues.