Evolutionary biology has long sought to understand the evolution of adaptive phenotypes. While purely genetic factors are often emphasized, environmental influences also significantly contribute to phenotypic variation. In this dissertation, we investigate the environmental regulation of visual sensitivity in cichlids. Cichlid fishes exhibit a diversity of color vision facilitated by differential expression of opsin proteins, including light-induced phenotypic plasticity which was known to occur during development. To determine whether cichlids retain plasticity into adulthood, adult fish were moved between different light environments. Adult fish not only exhibited plasticity of opsin expression, but it was rapid, reversible, and shifted in a direction that would maximize light input to the retina. In order to observe the effect of diurnal variation of light on opsin cycles, we collected retinae from a continuous 25 hour time-series. This enabled us to develop guidelines for qRT-PCR normalization and determined that opsins were most highly expressed in the late afternoon. Opsins also cycled together, suggesting some common diurnally-sensitive regulatory mechanism. Given that there are species differences in plasticity, we sought to determine genetic loci that were affected by light. To determine how quantitative trait loci (QTL) changed between light environments, we raised all generations of a previously studied hybrid cross in a new light environment. One QTL was significant in both environments, indicating it is universally important for opsin expression. However, another QTL was only significant in one light environment, indicating it may be environmentally-sensitive. We propose a possible candidate gene for this locus. Plasticity is prevalent during development, with thyroid hormone (TH) being a common cue for developmental changes. Previous studies have found TH causes opsin expression shifts from short- to long-wavelength sensitivity as organisms age. To see its effect on species of cichlids with different developmental opsin progressions, we treated fish with TH. Interestingly, TH shifted fish to longer-wavelength sensitivities regardless of species. We also uncovered a possible interaction between TH and light-induced plasticity. Characterization of environmental influences on vision will continue to enhance our understanding of the evolution of visual diversity in these incredible fishes.