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<i>Anopheles merus</i> can breed in a range of saltwater concentrations. The consequences of this ability on the life history of adult <i>An. merus</i> are poorly understood. This study examined the effects of exposure to 0, 2.1875, 4.375, 8.75, and 17.5 g/L of sodium chloride on <i>An. merus</i>. The effects on larval development, adult longevity, fertility, and fecundity, as well as deltamethrin tolerance were examined. The effect of larval salt exposure on the expression of <i>defensin-1</i> in adults was examined by quantitative Real-Time PCR. Finally, the effect of the larval salt concentration on microbial dynamics was assessed by 16S Next Generation Sequencing. High concentrations of saltwater increased larval development time and number of eggs laid, as well as deltamethrin tolerance. Larval exposure to salt also reduced the expression of <i>defensin-1</i>. The exposure also had a significant effect on microbial diversity in larvae and adults. The diversity of larvae decreased once adults emerged. Salt-tolerant bacterial genera predominated in larvae but were absent in adults. High salt concentrations resulted in greater abundance of <i>Plasmodium</i>-protective genera in adults. Although this study was conducted on a laboratory strain of <i>An. merus</i>, these data suggest that osmoregulation has a significant effect on the life history of the species with potential epidemiological consequences.