Lostroh scored 89.5 points to win the $5,500 cheque to top the Pool A money earners with $12,500.
The Colorado native hadn't planned on attending the Stampede but a last-minute phone call changed his mind.
"I was going to the short round of Greeley's (Colorado) rodeo on Thursday, and at 11 a.m. they called me and said, 'Hey can you be in Calgary tomorrow?,'" said Lostroh. "So we rode in Greeley, then jumped in the truck, drove all night, and got here just in time to ride."
World champion bareback rider Kaycee Field won his event with a 90.5-point ride for his first victory of the Stampede after three events.
"I had a slow fourth (of July run) and the first few days up here, I've been trying to muscle through it a little too much," said Field, who leads the standings with $10,000. "This horse really helped me out loosen up and have a little more fun, instead of trying to be so serious and strong."
In the tie-down roping, Tuf Cooper and Bradley Bynum finished their runs in just 6.9 seconds to tie for first and win $5,000 each.
There was also a tie in steer wrestling. Casey Martin and Wade Sumpter each finished in 3.7 seconds to share $5,000 each.
Meanwhile, barrel racer Mary Walker finished first in 17.63 seconds, and in saddle bronc Cody DeMoss won with 89 points. Rylan Geiger of Bracken, Sask., finished third with 87 points and leads the Pool A standings with $14,000.