Roundup Fishing Access Site is on river-right of the Blackfoot River on the downstream side of the Highway 200 bridge at Ninemile Prairie Road. The site is just downstream of Roundup Rapid, one of the Blackfoot's biggest, and features a vault toilet and a slide launch for rafts.
Day-boaters usually put in at Russell Gates (12 miles upstream) or Sperry Grade (10 miles upstream) to take out at Roundup, or put in at Roundup and float 6 miles to Corrick's River Bend FAS (which has nice camping) or 10 miles to the Whitaker Bridge access. All these stretches offer abundant Class II whitewater, and at high flows the narrow river is a nonstop ride.
Note that the dirt Ninemile Prairie Road cutting off of Highway 200 at Roundup continues along the river to Whitaker Bridge, where it becomes Johnsrud Park Road until its re-intersection with Highway 200 at Johnsrud Park.
Roundup is 29 miles upstream of the Blackfoot's mouth in Bonner, and 25.5 miles by road from Bonner on Highway 200.