The interfacial tension, gamma, between the coexisting isotropic and chiral nematic phases of aqueous suspensions of cellulose crystallites was determined at very low ionic strengths by the sessile drop method as a function of the concentration of cellulose crystallites. Values for gamma were measured in the range 10(-3) to 10(-4) mN/m, increasing with crystallite concentration. The measured values are in the range predicted by theories based on Onsager's second virial approximation when the equivalent hard-core dimensions of the rods are estimated from the phase separation data. However, the values for gamma strongly depend on the concentration of particles, in contrast to theoretical predictions. The polydispersity in particle dimensions may contribute to this discrepancy.