Called up from Double-A Tennessee, Castro became the first player to record six RBIs in his first Major League game, hitting a three-run homer in the second inning and a three-run triple in the fifth inning of a 14-7 victory against the Reds.
The previous high was five RBIs, recorded most recently by Oakland's Ben Grieve on Sept. 3, 1997, against San Francisco, according to research by Elias Sports Bureau.
Castro, who was batting .376 at Tennessee with one home run, came up with two on and none out in the Chicago second and connected on a 2-2 pitch by Homer Bailey. The ball landed in the right-field bleachers and was quickly retrieved.
It was quite a first day on the job for the shortstop, who got a 7 a.m. wakeup call that he was headed to the big leagues.
Castro is the sixth Cubs player to homer in his first at-bat and the first since pitcher Jim Bullinger did so on June 8, 1992. He also is the third rookie this year to homer in his first at-bat. The Cubs saw the Braves' Jason Heyward do it on April 5 in Atlanta off Carlos Zambrano. Minnesota's Luke Hughes did it on April 28 at Detroit.
The Cubs loaded the bases in the fifth, and Micah Owings, on in relief of Bailey, walked Geovany Soto to force in a run. Castro then lined a triple to left-center for three more RBIs and a nine-run lead.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.