A cartridge monitoring and display system for an automatic or semiautomatic firearm. A programmed microcontroller receives input signals from each of a slide switch and a magazine switch, which switches generate respective signals upon actuation of the firearm slide and upon insertion or withdrawal of a cartridge magazine. The microcontroller calculates the number of rounds remaining in the firearm and displays the number on an LCD display. A visible and audible warning is given when the cartridge count drops below a predetermined minimum level. The entire system is incorporated in a mounting strap which may be inserted between the frame and hand grip of a conventional semiautomatic pistol.