Audio-visual integration was studied in children aged 8-17 (N = 30) and adults (N = 22) using the "flash-beep illusion" paradigm, where the presentation of two beeps causes a single flash to be perceived as two flashes ("fission" illusion), and a single beep causes two flashes to be perceived as one flash ("fusion" illusion). Children reported significantly more fission illusions than adults, indicating that auditory and visual information was integrated more often, and less selectively, than in adults. Within either group, illusion reports did not correlate with either age or motor coordination measures. The current results show that the form of multisensory integration indexed by the illusion is slow to mature in normally developing children.