Methods, program products, and systems for multi-tier detection of a geofence are disclosed. In general, in one aspect, a mobile device can be configured to perform a task when the mobile device enters a geographic region. The mobile device can monitor a current location using a multi-tiered approach. A baseband subsystem can monitor a coarse location of the mobile device using a CDMA system identifier, a CDMA network identifier, a CDMA zone identifier, or a CDMA base station identifier, in that order, as the mobile device moves closer to the geographic region. The baseband subsystem can notify an application subsystem when the mobile device is in a cell that intersects the geographic region. The application subsystem can perform the task upon notification.