Determination of the atomic sites for the doped niobium atoms by the X-ray diffraction method has been undertaken to obtain the structural basis for the understanding of semiconductive behavior of crystals. Diffraction intensities were measured with spherical samples ground from the undoped and doped (4 mol. %) TiO2 single-crystal rods grown by the flame-fusion method. Measurements were carried out by an automatic single-crystal diffractometer and observed intensities were corrected for Lp and absorption factors. The total number of the independent (hk0) reflections was 45 with Mo Kα. radiation. Structure factors were calculated on the baiis of the following atomic arrangement: titanium at equipoint (2a) of space group P42/mnm, O at (4f) with x = 0.306, and an isoatomic temperature factor B = 0.70 were used. Reliability factor R for the undoped rutile was 0.037 without further refinement. For the doped crystal, an R-yalue of 0.053 was obtained for the substitutional model, where niobium was assumed to be at x = 0, y = 0. The R value was 0.125 for the interstitial model, where niobium was assumed to be at x = ½, y = 0. The difference Fourier map was obtained by subtracting the electron density distribution in a Fourier map of the undoped crystal from that ot the doped crystal. Since the intensity measurements were done only on a relative scale, the two Fourier maps were normalized so as to give an identical number of electrons under the oxygen peak. The resulting map revealed a small peak only at the location x = 0, y = 0. It could be concluded that niobium atoms enter the titanium sites substitutionally, at least in rather heavily doped crystals. Further work on the refinement of the doped structure is in progress.