The first card skimming device was reported on Tuesday at the Chevron station at Ygnacio Valley Road and North Main Street by an alert clerk, police said.
Police seized the device, which they believe had been installed only hours before.
On Wednesday, a second device was found attached to a pump at the Valero gas station at Ygnacio Valley Road and North Broadway. Police believe it had been installed in the previous 24 to 36 hours.
Both devices were of the same design and construction, and were installed inside the pump with no external indications they were attached, police said.
Investigators are examining surveillance video from both stations.
Patrons of these stations that saw anyone tampering with the pumps, or citizens who see any suspicious activity at any gas station, should contact police at (925) 943-5844.
Patrons of these stations should also monitor their credit reports for any unauthorized credit activity.
All gas stations in the area have been contacted and asked to check their pumps on a regular basis.