OAKLAND -- When Jason Giambi signed with the A's in January, he was sitting on 396 career home runs and talked openly about wanting to play long enough to reach 500.
No. 400 came in the fourth inning Saturday night.
Mired in one of the worst slumps of his career, Giambi belted a belt-high fastball from Diamondbacks right-hander Dan Haren into the right-field bleachers during the second game of a three-game series at Oakland-Alameda County Coliseum.
"I was glad the fans here got a chance to see it where I started my career," said Giambi, who was drawn out of the dugout for a curtain call. "The fans have always been great to me here."
The 44th player in big league history to hit 400 career homers, Giambi entered the game batting .176 (18-for-102) over his past 30 games.
Giambi returned to Oakland, where he spent the first seven years of his career before playing for the Yankees for seven seasons, on a one-year deal with a club option for 2010.
He hit 209 homers with New York, and his 191 homers for the A's put him in a tie with Gus Zernial for ninth-most in club history.
Mychael Urban is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.