We photographed the pupils of 128 normal subjects in dim light, morning and afternoon, for five consecutive days. Fifty-two of the subjects (41%) had an anisocoria of 0.4 mm or more at one time or another during these five days, but only four (3%) had unequal pupils of 0.4 mm or more in all ten photography sessions. At any given examination, a fairly constant 19% (24 of 128) of the subjects showed this amount of anisocoria. These numbers shifted dramatically when anisocoria was defined as a pupillary inequality of greater than, or less than, 0.4 mm. The prevalence of anisocoria did not vary with the time of day, from day to day, or from week to week, nor was it influenced by the sex, age, or iris color of the subject.