Stephen Strasburg gave up four runs in six innings of work, and Tom Koehler and two Miami relievers shut down an inconsistent Nats offense again in an 8-0 win on Saturday.
Strasburg (1-2) struck out six and walked two. His ERA in 2015 is at 4.68 after five starts.
Washington only had eight baserunners for the game, with seven hits and a base on balls.
Giancarlo Stanton had three hits for Miami, including his fifth homer of the season.
The Nationals have now dropped four in a row after climbing back to .500 earlier in the week.