The assumption that vergence adaptation to a prism could only take place during single binocular vision was investigated by having subjects wear a prism while fusion was dissociated. Adaptation was evaluated by its effect on lateral heterophoria. Dissociation and measurement of heterophoria were by means of an automated version of Duane's screen and parallax test. A preliminary investigation with 37 subjects fixating at 6 m while wearing 4 delta base-out and with 33 other subjects fixating at 50 cm while wearing 3 delta base-in showed that at least 25% adapted to the prism. Results of an investigation with 2 subjects fixating at 6 m (4 delta base-out and base-in) and with 3 subjects fixating at 50 cm (3 delta base-in) are presented as evidence that prism adaptation can take place. It was also found that the power of the added prism could just as well be provided by the phoria test's variable prism. In other words, there is a possibility that adaptation will occur when heterophoria is measured with a variable prism.