The root ultrastructure of seedlings grown in anaerobic conditions was investigated in four neotropical species: Sesbania virgata, Erythrina speciosa, Sebastiania commersoniana (all present in waterlogged or flooded areas), and Schizolobium parahyba (that occupies mainly dry areas). Anaerobiosis induced an increase in the size of mitochondria, dilatation of cristae and of the endoplasmic reticulum, and fragmentation or concentric arrangement of reticulum saccules. The ultrastructural alterations were reversible only for S. virgata and E. speciosa. The seedlings of S. parahyba and S. commersoniana were more sensitive to oxygen deprivation and presented extensive cell disruption. The results are discussed in terms of energy supply.22499-105