A comparative investigation of the frequency of photoparoxysmal responses (PPR) in the white, mixed and African populations of the Cape Peninsula is presented, based on 9697 consecutive referrals for EEG. PPR were observed in 2.5% of the white, in 1.3% of the mixed and in 0.9% of the African population groups. In the present material there were more patients with epilepsy in the mixed than in the white group, the frequencies changing with age in the two groups. Minor interrelationships of PPR exist with sex, age and race considered separately. Another minor relationship could be demonstrated between PPR and epilepsy. Three-factor interaction exists among epilepsy, age and race, and a further three-way interaction could be shown among sex, age and epilepsy. These relations and interactions were all significant below the 1% level. Standard EEG records (including response to hyperventilation) sensitivity limits to PPR, and the types of seizures themselves, did not differ significantly between the two main ethnic groups.