The authors hypothesised that if vertical vergence were mediated by vertical rectus muscles, then the eye that moves during a vertical fusion task would produce larger vertical vergence when abducted versus adducted. Similarly, if mediated by the oblique muscles then the effect would be greater when the eye is adducted than abducted. Thus, the authors compared vertical fusional amplitudes and vertical vergence responses at different horizontal orbital eye positions in two primates in order to isolate the muscles that mediate vertical vergence. Measurements from both monkeys showed that vertical fusion capability differed with orbital eye position. The results indicated that different muscle groups mediated vertical vergence in the two eyes of each of the monkeys. This confirms that different subjects adopt different strategies to generate vertical vergence and it seems likely that both vertical rectus and oblique muscles are involved in vertical vergence with different relative contributions of these muscles.