Poeciliids have been identified as one of the 100 most invasive species in the world, as well as a “species of concern” in the states of Hawai`i, Washington, Oregon, and New Mexico. Despite their global prevalence, few studies have quantified their impacts on stream ecosystem structure, function, and biodiversity. This project documented the effects that poeciliids are having on Hawaiian stream ecosystems. Comparisons made between poeciliid and poeciliid-free streams in nutrient dynamics, benthic algal biomass, invertebrate species composition, and fish community structure on Hawai`i Island revealed that invasion by exotic poeciliids is significantly altering stream ecological structure and function. Significant differences between stream types were found for sum nitrate (∑NO₃ ⁻ = NO₃⁻ + NO₂ ⁻), total nitrogen (TN), and total organic carbon (TOC) yields, ash-free dry mass (AFDM), Hydroptila, Dugesia, and chironomid percent contributions, and invasive and native fish densities. Sum NO₃⁻, [TN], and [TOC] yields were 8x, 6x, and 5x times higher, respectively, in poeciliid streams than in poeciliid-free streams. Benthic algal biomass (AFDM and chlorophyll a) was 1.5x higher in poeciliid streams than in poeciliid-free streams; diatoms dominated algal communities in both stream types. Invertebrate communities were dominated by Chironomidae (midgefly larvae) in both stream types; Chironomidae densities were 1.6x lower in poeciliid streams. Poeciliid streams also had decreased hydroptilids (microcaddisfly larvae) and increased Dugesia (flatworms) densities. Additionally, mean densities of native gobies were 2x higher in poeciliid-free streams than in poeciliid streams. In poeciliid streams, poeciliid densities were 7.6x higher than densities of native fish. All of these results suggest that poeciliids are impacting the ecological structure, function and biodiversity of Hawaiian streams and underscore the need to restore invaded streams and to prevent future invasions.