The evidence for the existence, nature and degree of physiological exophoria was examined with respect to a sample of 663 optometric patients. It was found that the mean value for heterophoria is the same for distance and near vision up to the age of the early twenties. After this age, the mean value for distance heterophoria remains unchanged throughout life whilst there is a change in the value for near vision towards exophoria, giving 5 delta physiological exophoria. No differences between males and females were found. Physiological exophoria is shown to increase regularly with age and no sudden increase was found at the age when presbyopic additions were first worn. It is concluded that presbyopic additions are not the cause of the physiological exophoria.