Tropical island streams worldwide are threatened by existing or proposed dams and diversions that remove freshwater for human use; yet, there have been few studies that address effects on aquatic communities.  The objective of this study was to quantify changes in tropical macroinvertebrate communities associated with stream flow withdrawal.  Benthic macroinvertebrates were collected from riffle habitats located above and below a stream diversion on Maui, Hawaii, from June to August 2000.  Native and introduced taxa were identified from both locations. The most dominant taxon was midges (Diptera: Chironomidae) followed by two introduced caddisflies, Cheumatopsyche analis (Banks) (Trichoptera: Hydropsychidae) and Hydroptila potosina (Buenoa-Soria) (Trichoptera: Hydroptilidae).  A native shrimp, Atyoida bisulcata, and beach fly, Procanace sp. (Diptera: Canacidae), were either eliminated from, or significantly reduced below, the diversion.  Mean total macroinvertebrate densities were significantly greater above (13,357 individuals/m2) the diversion compared to below (10,360 individuals/m2).  Mean total macroinvertebrate biomass was significantly reduced by 60 percent below the diversion, but specific taxa did not show this effect. These results suggest that diverted stream flow limited macroinvertebrate colonization and growth, expressed as reduced community density and biomass, which may alter the structure and function of other trophic levels within tropical stream food webs.