We describe the results of experimental investigation of the third-harmonic generation (THG) in -BaB 2 O 4 crystal pumped with a femtosecond laser radiation and cut at the angle  S permitting a process of direct phase-matched generation of oooe type. A theory of direct THG in uniaxial crystals, which assumes that the effective nonlinear coefficient has an additive nature with respect to the contributing processes, has been used when interpreting the experimental data. The angular dependence of the third harmonic intensity and its dependence on the incident pump power have been analysed in order to ascertain the process, the contribution of which is dominant. The analysis of our experimental results obtained under the condition of pumping  -BaB 2 O 4 ( S = 53) sample with femtosecond pulses ( = 800 nm,  = 70 fs, and max P = 100 GW/cm 2 ) has proven that the contribution of second-order cascaded processes to the THG is prevailing.