Abstract
I studied sexual behavior of immature bonobos (Tan paniscus) in a wild group living at Wamba, Zaire, with special reference to its development. Even immature individuals under 1 year old performed sexual behavior. Sexual  behavior occurred in almost all age-sex combinations, except between immature and mature females. Based on analyses of behavioral pattern and context, I classified sexual behavior involving immature individuals into three categories. 

1. Genital contact between immature individuals was observed during play, and was performed by males more frequently than by females. This sexual behavior shared many traits with that of other great apes.
2. Copulation-like genital contact was observed between immature males and mature females. Its frequency increased with the immature male's age; it developed into copulation in adulthood.
3. Genital contact used to regulate interindividual relationships.

This behavior, which is unique to bonobos, was absent among infants. It developed between late juvenile and early adolescent periods in association with changes in social circumstances.

Keywords
Pan paniscus; development; sexual behavior; genital contact; social behavior