Abstract: The self-contained ball-strike detector uses two transducers to detect the presence of an incoming pitch, and a series of transducers located on the upper surface of a home plate-shaped housing to determine whether the pitched ball is within the strike zone. Ultrasonic transducers are located near both the right and left boundaries of the strike zone. These transducers and a centrally-located transducer emit high frequency signals in the direction of the pitched ball. A reflected signal is used to determine whether the pitched ball is within the strike zone. The size of the strike zone may be changed to accommodate batters of different heights. The apparatus includes audio and visual indicators of whether the pitch is a "ball" or a "strike" as well as indicators if the batter is "out" or is entitled to a "walk". The apparatus maintains the ball/strike count for each batter, and has light emitting diodes to visually indicate the current count for the batter.