Within the mosquito genus, Ochlerotatus there are several species that can survive in a wide range of salinities (up to 3X the concentration of seawater) and others that are restricted to freshwater. Both freshwater and saline-tolerant forms are osmoregulators and the difference in their osmoregulatory capacities is a morphological distinction in their rectum. The freshwater species use their rectum to take up ions from the luminal contents while the saline-tolerant species use an extra-rectal segment to secrete ions, thereby producing highly concentrated urine. We are the first to describe the osmoregulatory status of several species within this genus by performing a salinity assay and examining the rectal morphology. To determine the pattern of evolution of saline-tolerance we mapped this trait on a phylogenetic tree we constructed using rDNA genes as molecular characters. Our results suggest saline-tolerance is homoplasic in Ochlerotatus mosquitoes; it either evolved independently on more than one occasion or evolved once and was subsequently lost.