The Exterminator is essentially a clone of the arcade game Centipede. A centipede forms from the top of the screen and gradually moves down it, changing direction when striking the many barriers it hits. If it reaches the bottom of the screen, it wins. You control a small droid, which can move up the screen shooting out the centipede, cutting it shorter each time). You can also shoot the barriers, which take multiple hits to remove - the less barriers there are, the longer it will take for the centipede to reach the bottom.
Occasionally a phoenix will appear, either moving horizontally towards the bottom of the screen, or vertically down from the top, to shoot for a points bonus or avoid. You have three lives, but on losing one you gain points for any remaining pieces of centipede. The high score is updated constantly once you pass it, and (unusually for the time) an attract mode demos the game.