Home runs by Gary Carter and Warren Cromartie carried the Montreal Expos to a 6-4 victory over St. Louis in the first game of a doubleheader today, then Cardinal Manager Ken Boyer was fired before the second game.

The firing announcement came from St. Louis, where August Bush Jr., team chairman and president, named Whitey Herzog as Boyer's successor.

Herzog guided Kansas City to three successive Americanu League West titles during 1976-78 but was dismissed by the Royals after the 1979 season when they finished second three games behind California.

With Jack Krol, third base coach, directing the Cards, they lost again in the second game, 9-4. Andre Dawson stroked five hits as the Expos won their sixth straight.

Dawson doubled off Roy Thomas (1-3) in the seventh for two runs after Ron LeFlore singled and Rodney Scott was hit by a pitch. The three run burst wiped out a 3-2 Cardinal lead.

The first-game loss was St. Louis' 21st in 26 games and fourth in a row. With that opening-game loss the Cards stood 15 games below .500 and last in the National League East, 11 1/2 games behind the front-running Expos -- although the Cardinals lead the league in batting.

"I'm going to hold my head up high," Boyer said. "I don't believe they can tell me there are problems on this team.

"I don't think there was anything I could do to help improve this team. They have asked me to stay on in some capacity but I haven't made up my mind."

Boyer, 49, former all-star third baseman and one of the most popular Cardinal players, became the third St. Louis manager in three years on April 29, 1978, when he was named to replace Vern Rapp only 18 games into the season.

Boyer led the Cardinals to a 69-93 record in 1978, the team's worst finish since 1924. Last year the Cardinals improved 17 games and finished third in the National League East with an 86-76 record, their best mark since 1974.

At a St. Louis news conference, Herzog said he expects only two things from the players: that they be at the ball park n time and work hard while in uniform.

"I'm going to tell them (the players) that it all boils down to the fact that I'm easy to get along with if they work hard.

"If you're good enough, you'll win," Herzog said. Ballplayers make the manager. I'm not that smart."

"I'm going to rely on all the coaches a lot. I'm going to talk to those coaches and I'm going to pick their brains," Herzog said.

"But I'm opinionated and hard-headed. Once that game starts, I'm the boss."