A high total pneumocandin titer (B-0 + C-0) with a low percentage of the structural isomer pneumocandin C-0 was achieved by carrying out fermentations of Glarea lozoyensis at a high residual fructose concentration (125 g/l initial). When the fermentation was carried out at a low residual fructose concentration (40 g/l initial), pneumocandin production increased by 34%. However, a disproportionate increase in the level of pneumocandin C-0 synthesized (250% increase vs 30% increase for pneumocandin B-0) was observed. Midcycle addition of 150 mM NaCl or 116 mM Na2SO4 to low residual fructose fermentations returned the titer and isomer levels to those seen for the high residual fructose fermentation. The increase in pneumocandin C-0 synthesis under low residual fructose conditions appears to be linked to the increase in the synthesis of trans-4 hydroxyproline, with the synthesis of trans-3 hydroxyproline remaining unaffected. This suggests that the formation of pneumocandin C-0 is the result of a misincorporation of trans-4 hydroxyproline instead of trans-3 hydroxyproline by the pneumocandin peptide synthetase, and that the amount of trans-4 hydroxyproline formed dictates the frequency of this misincorporation.