Pitching coach Rich Dubee said Halladay threw 56 pitches, stopping to simulate rest between innings. He could have several more bullpen sessions before pitching a few simulated games.
The two-time Cy Young Award winner has been out since May 28 because of a strained latissimus dorsi muscle on the right side of his back. The Phillies are 11-21 since he last pitched, a tailspin that has the organization talking about possibly being sellers at the July 31 non-waiver trading deadline.
Halladay is an uncharacteristic 4-5 with a 3.98 ERA before.
"I don't know where he'll be when he gets back," Dubee said. "I hope he's close to what he used to be. But the signs are leading that he's doing very well."