PhillyCarShare has vehicles stationed around the city and charges users hourly or daily rates, plus a per-mile fee. Renters used the gas cards to fill up as needed.
The thefts have occurred sporadically in parking lots used by the program, but investigators were focusing their efforts in South Philadelphia, police Lt. Frank Vanore said.
In an e-mail to members, PhillyCarShare said it was working on a system to keep the cards secure. In the meantime, employees are filling up the cars daily.
The nonprofit asked its members to be patient.
"In the event you do find yourself in a car that's running low, chances are we are already in route to fill 'er up," the e-mail said. "A permanent solution is coming very soon."
Cars now have window stickers that say, "This car does not contain a gas card."
A spokesman for PhillyCarShare did not immediately return a telephone call from The Associated Press on Friday.