Conditions for optimizing spore production, especially chlamydospores, by host-specific mycoherbicidal strains of Fusarium oxysporum causing vascular wilts in coca (Erythroxylum coca) and poppy (Papaver somniferum) were studied in 2.5-1 fermenters. The fermentor dissolved oxygen and pH had significant effects on the growth characteristics of F. oxysporum strains. The effect of the fungal strain, however was not significant for most of the variables studied except for chlamvdospore formation. After 14 days of fermentation, the spore types produced were microconidia and chlamydospores, with very little production of macroconidia. While the total viable counts were significantly higher under high than under low dissolved O-2, the chlamydospore counts were significantly higher under low than under high dissolved O-2 The percentage of chlamydospores obtained, as a proportion of total viable was significantly higher when the fermenter pH was increased, than when it was not. Scaling-up the liquid fermentation to 20 1, yielded log(10) c = 6.5 (where c = chlamydospores ml(-1)) after 14 days＇ fermentation, with biomass viable counts of log(10)v similar to 8.0 (where v = viable counts g(-1) air-dried biomass). A single-step liquid fermentation reported in this study increased chlamydospore yields and reduced the time required for their production with techniques currently available from 5 weeks to less than 2 weeks.