Ausmus will be paid a base salary of $2 million but can make more in incentive bonuses based on playing time.
"If his role were to mirror what it's been the last couple years, his salary will be raised closer to what it was in the past," general manager Ed Wade said.
Ausmus earned $4 million in 2006 and $3.5 million in '07.
The plan is for Ausmus, 38, to play on a part-time basis, with Towles catching the majority of the games. Should Towles struggle, however, the Astros can turn to Ausmus, who has played in at least 130 games in four of the past six years.
"Brad is an established veteran catcher with the ability to play a lot," Wade said. "The mindset is if we can get 20 more points on the batting average [from the No. 1 catcher] or get a guy to knock in 20 more runs, and we have Brad, who has a great presence behind the plate and who handles pitchers so well, we think we've got a pretty complete package going at that particular position."
Ausmus is fully aware of what his role will be in '08 and said he is not worried about how much playing time he will ultimately receive.
"I'll let Coop [manager Cecil Cooper] decide who catches when," Ausmus said. "I'll just follow orders. I don't know how Coop envisions it and I'm sure anything he envisions is probably subject to change. I'll be ready."
Ausmus sees a great upside in Towles, who hit .375 over 14 games after earning a September callup. Fourteen games is hardly a large sampling, however, and that's where Ausmus' experience can help.
"As a receiver, he's very good," Ausmus said. "He's probably one of the better receivers in the league. But just arriving to the big leagues, there's some things he has to learn. It comes with experience, and he doesn't have experience. Hopefully he'll speed up the learning process, in theory, with me around."
Ausmus hit .235 with three home runs, 17 doubles and 25 RBIs over 117 games with the Astros in 2007. He recorded his 100th career stolen base on July 27th, becoming the 21st catcher all-time to reach the 100-steal plateau.
Defensively, he ranked fourth among National League catchers with a .995 fielding percentage, committing only four errors in 814 total chances. He made his franchise-record eighth Opening Day start at catcher for the Astros in 2007, breaking a tie with Alan Ashby.
Ausmus doesn't know if 2008 will be his final year as a Major League player, but he does know it'll be his last year in Houston. He lives year-round in San Diego and has decided that if his career does continue, he will stay close to home and to his wife, Liz, and their daughters, ages 9 and 8.
"Large chunks of time away from home is not in the best interest of my family," Ausmus said, adding that his wife and kids were on board with him playing in Houston in 2008.
Ausmus was one of six Astros players to file for free agency on Monday, the first day players were eligible to do so. Infielders Mike Lamb and Mark Loretta, outfielder Orlando Palmeiro, left-handed reliever Trever Miller and right-handed reliever Brian Moehler also filed. Jason Jennings filed for free agency on Tuesday.
Wade hasn't ruled out re-signing any of the Astros free agents at this point, but it's safe to speculate the club is more interested in retaining Loretta than the others. Loretta, a veteran second baseman, could provide a solid option at a position that will likely be manned by Chris Burke in '08.
"We haven't closed the door on any of our guys at this point," Wade said. "With the things going right now from a staffing standpoint, it was prudent for us to let them go file and see where we are."
Alyson Footer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.