Teenagers make up a quarter of all mothers in Transkei, South Africa, and well over 75% of them are unmarried, Such a high rate of teenage pregnancy is indicative of a high level of unprotected adolescent sexual activity, We examined sexual behaviour, knowledge and attitudes to sexuality among adolescent school girls in Transkei, using a self-administered questionnaire, in order to establish the incidence of sexual activity, and associated risk factors, Of the 1072 respondents, 74.6% were already sexually experienced, and 21.0% were not, The majority of sexually experienced girls (SEGs) and sexually inexperienced girls (SIGs) were living with both their parents, There were no religious differences between the two groups of girls, The age of SEGs at first coitus correlated positively with the age of menarche, and the age at the first date, suggesting that sexual maturation and onset of dating were possible risk factors for initiation of sexual activity, Contraceptive use was low, and a third of SEGs had been pregnant at least once. The knowledge of reproductive biology among both groups of girls was generally poor, although SEGs were significantly more knowledgeable than SIGs, The majority of girls in both groups did not approve of premarital sex, and adolescent pregnancy. They also did not approve of the idea of introducing sex education in schools, or the provision of contraceptives by schools. Nearly a third of the respondents in both groups did not wish to get married in future, In conclusion, there is a high level of unprotected sexual activity among school girls in Transkei. The risk factors for this include early sexual maturation, early onset of dating, and poor knowledge of reproductive biology and contraceptives.