This study examined benthic macroinvertebrate assemblages and environmental parameters along a longitudinal gradient in Fort Peck Lake, Lake Sakakawea, and Lake Oahe.  Reservoirs were divided into upper, middle, and lower zones based on water depth and distance from the dam, and a probability-based sampling design was used to select sites in both main stem and bays.  Benthos was collected with a PONAR dredge.  For all reservoirs combined, mean (± SE) total abundance of benthic macroinvertebrates was 2291 ± 642 /m² in the upper reservoir zone, 1100 ± 88 /m² in the middle zone, and 1836 ± 341 /m² in the lower zone.  Densities of both oligochaetes and chironomids were reduced in the middle zone relative to the upper and lower zones.  Chlorophyll a concentrations were greater in the upper zone (5.0 ± 0.7 µg/L) relative to the middle and lower zones (2.0 ± 0.1 and 1.5 ± 0.1 µg/L, respectively).  Information on the abundance, distribution and variability of benthic macroinvertebrates will be used to further refine assessment strategies for large reservoirs.  This abstract does not necessarily reflect USEPA policy.