A method, based on the negative adsorption of NO2 -, has been developed to determine surface area of roots. Young roots of 3–4 year old plants ofAcacia catechu, Eucalyptus camaldulensis andLeucaena leucocephala were up-rooted and cut into 18 cylindrical pieces. Each root piece was immersed individually for 10 sec in 0.05M, 0.10M and 0.15M aqueous NaNO2 solution and the excess solution on the root surface allowed to drain off. It was then transferred to conical flask containing distilled water and shaken for 15 min for desorption of nitrite. A known quantity of this aliquot was reacted with 1% acidic sulphanilamide and 0.02% NED HCl. A pink colour developed, and its optical density was read at 540 nm. A positive linear correlation was noted between colour density and root surface area. The respective correlation coefficientvalues for 0.05M, 0.10M and 0.15M NaNO2 solutions were 0.973, 0.963 and 0.964 forAcacia catechu, 0.933, 0.903 and 0.898 forEucalyptus camaldulensis and 0.968, 0.976 and 0.972 forLeucaena leucocephala (significant atp<0.001). The method was successfully adopted to determine the root surface area of seedlings ofAlbizia lebbek, A. procera, Acacia auriculiformis, A. nilotica, Dalbergia latifolia andD. sissoo.