Plants of Avicennia marina (Forssk.) Vierh. Var. Australasica (Walp.) Moldenke ex N. C. Duke that had been continuosly flooded for about two months produced numerous downward-growing roots well above the soil surface, as they sometimes do after pollution events in the field. These downward-growing roots (stilt roots) were not produced by the non-flooded plants. The stilt roots generally resembled inverted normal pneumatophores, and had many lenticels on their surface. We suggest that a stimulus for stilt root production in this species may be an increase in ethene (ethylene) resulting from restricted ventilation. We suspect that threre is a requirement for some availability of oxygen to support the growth of new ventilating root structures, and that this criterion is satisfied near the air-water interface. The number of aerenchymatous organs including all types was the same in both flooded and non-flooded plants, and we speculate that this number may be pre-determined in some way in plants of this size.