A study of the structure and floristics at a transition zone from a monodominant to a more diverse forest in the African humid tropics was conducted to elucidate the mechanisms maintaining floristic diversity and the discontinuity between mixed forests and forests dominated by a single tree species (monodominant). The mixed forest's greater diversity could not be explained by substrate differences, greater maturity, or greater predation on seeds or juveniles. The dominant species of the monodominant forest was shade-tolerant and had poorly dispersed seeds. Tree species associated with the dominant were also found in the mixed forest. Monodominant and mixed forests occur side by side in the Asian and American tropics as well. As in the African example, many of these monodominant forests share most species with neighboring mixed forests. Characteristically, the dominant species have large seeds and shade-tolerant seedlings. Monodominant tropical forests are widespread and may indicate areas that have not ex...