The (a,b) plane angular dependence of the third-order nonlinear optical susceptibility, χ(3), of a c-cut paratellurite (α-TeO ₂) single crystal was quantitatively evaluated here by the Z-scan technique, using a Ti:sapphire femtosecond laser operated at 800 nm. In particular, the mean value Re(χ(3)-a,b)(α-TeO ₂) of the optical tensor has been extracted from such experiments via a direct comparison with the data collected for a fused silica reference glass plate. A Re(χ(3)-(a,b)(α-TeO2)):Re(χ(3))(SiO2 glass) ratio roughly equal to 49.1 is found, and our result compares thus very favourably with the unique experimental value (a ratio of ∼50) reported by Kim et al. [J. Am. Ceram. Soc. 76, 2486 (1993)] for a pure TeO ₂ glass. In addition, it is shown that the angular dependence of the phase modulation within the (a,b) plane can be fully understood in the light of the strong dextro-rotatory power known for TeO ₂ materials. Taking into account the optical activity, some analytical model serving to estimate the diagonal and non-diagonal components of the third order nonlinear susceptibility tensor has been thus developed. Finally, Re(χxxxx(3)) and Re(χxxyy(3)) values of 95.1×10-22 m2/V2 and 42.0×10-22 m2/V2, respectively, are then deduced for a paratellurite single crystal, considering fused silica as a reference.