Rice cv. IR8 was grown in flooded Maligaya soil contained in pots. Plants were exposed to 15N in controlled environmental chambers for 7 days at 49 days age. The roots and rhizospheric soil contained large 15N conc., while the upper leaves had small but significant 15N conc. In a similar study with cv. IR26 grown at IRRI in the field and subsequently transferred to sol. culture placed in chambers supplied with 15N for 7 days, the greatest 15N conc. was in the outer leaf sheaths, though some enrichment occurred in the lower rice stems. Heterotrophic molecular N fixation by blue-green algae was suspected, since this alga grew on submerged plant parts of flooded rice in the field.