Abstract Although rice has long been recognized to be uniquely adapted for growth in low oxygen environments of flooded rice fields, rice weeds of the Echinochloa crus‐galli complex appear to be at least as well specialized for germination and growth under such unusual biological conditions.

Seeds of two varieties of E. crus‐galli germinate and grow for prolonged periods in a totally oxygen‐free environment. E. crus‐galli germinates as well as rice (Oryza sativa) under a total nitrogen atmosphere and produces as large a seedling in spite of its much smaller seed size. Like rice, the seedlings of E. crus‐galli are unpigmented, the primary leaves do not emerge from the coleoptile and no root growth occurs without oxygen. Of particular interest is the ultrastructure of mitochondria from anaerobically‐grown seedlings. Mitochondrial profiles from the primary leaf of seedlings grown continuously in nitrogen are very similar to those grown aerobically. The size and shape of the mitochondria are similar and the cristae are numerous and normal in appearance. This is in sharp contrast to previous studies of other species which have reported that mitochondria were vesiculate and tended to lose their normal fine‐structure after similar periods without oxygen.

Finally, based on ultrastructure and 14C labeling studies, anaerobically‐grown seedlings are highly active metabolically, which may explain, at least for E. crus‐galli var. oryzicola, its ability to germinate and emerge from flooded rice fields.