The aim of the studies was to evaluate the influence of N and P concentrations on dynamic and yield of oxalic acid biosynthesis from post-refining fatty acids by Aspergillus niger XP in semicontinuous cultures at low pH 4,0. Three semicontinuous cultures were carried out with A. niger XP, in media containing post-refining fatty acids as the only carbon and energy source. Ability of oxalate biosynthesis in media containing different levels of NH₄NO₃ and KH₂PO₄ was evaluated. In all cultures ability of utilization of post-refining fatty acids and production of oxalic acid were observed, however with different dynamics and yield. Biomass concentration in cultures varied from 6,5 gdm^-3 in media containing 0,5 gdm^-3 NH₄NO₃ to 30,5 gdm^-3 in media containing 1,5 gdm^-3 NH₄NO₃. The highest concentration of oxalic acid 56,2 gdm^-3 was obtained in medium with 1 gdm^-3 NH₄NO₃ and 0,5 gdm^-3 KH₂PO₄. The highest yield 1,97 gg^-1 was obtained in 1st cycle in medium with 0,5 g/l NH₄NO₃. Volumetric oxalic acid production rate varied significantly from 1,8 do 7,0 gdm^-3d^-1.

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