Trees belonging to 30 tropical African and South American hardwood species were wounded annually. This made it possible, after the trees had been felled, to mark precisey the annual growth rings and determine their boundaries. These boundaries are always formed during the longest dry season and clearly express the rhythm of cambial activity. The appearance and nature of these growth rings vary according to genera rather than to types of climate. Trees belonging to 30 tropical African and South American hardwood species were wounded annually. This made it possible, after the trees had been felled, to mark precisey the annual growth rings and determine their boundaries. These boundaries are always formed during the longest dry season and clearly express the rhythm of cambial activity. The appearance and nature of these growth rings vary according to genera rather than to types of climate.