Abstract.-We compared two Denil fishways, located on the west (low velocity - 10 % slope) and east (high velocity - 20% slope) side of the Mannheim weir, Grand River, Ontario, for use by upstream migrating white suckers Catostomus commersoni and smallmouth bass Micropterus dolomieu. Mark-recapture and radiotelemetry were used to assess attraction and fish passage. Movement of 85 radiotagged fish was monitored continuously during spring and early summer of 1995 and 1996. Attraction and passage efficiency of white suckers at the west fishway was approximately 50 %, and 55 %, respectively. Attraction efficiency of white suckers at the east fishway was approximately 59 % and passage efficiency was 38 %. The attraction and passage efficiency of smallmouth bass at the west fishway was approximately 82 % and 36 %, respectively. At the east fishway, attraction efficiency of smallmouth bass was approximately 55% while passage efficiency was 33%. There was an exponential decline in the numbers of both species that used each fishway relative to water velocity. The maximum water velocities used by white suckers and smallmouth bass were 0.96 m/s and 0.99 m/s, respectively. Distracting flows near the west fishway appeared to affect attraction. Both fishways passed equal numbers of smallmouth bass per year, and smallmouth bass that used the east fishway were significantly larger than individuals that used the west fishway. In contrast, more than twice as many white suckers used the west fishway and these fish were significantly larger than those that used the east fishway. Differences in passage were related to burst and critical swimming speeds, and the use of velocity refugia within the fishways.