Herbicidal potential of CFCF (cell free culture filtrate) of Phoma herbarum (FGCCPH#27) against Parthenium hysterophorus was determined by shoot cut, seedling and detached leaf   bioassays. Maximum mortality was shown by the inoculum formulated with sucrose + Tween-20 followed by Tween-80. Triton-X was found to be highly inhibiting in its action. Maximum average leaf area damage (LAD) of 85% on the seventh day and 65% of leaf damage by fourth day was observed when treated with CFCF obtained from 14 day’s old fermented broth at 50% concentration followed by 75 and 100%. Maximum phytotoxicity was obtained from 14 day’s old fermented broth with sucrose + tween 20  0.5% as formulating agent. Chlorophyll and protein contents were also significantly affected when treated with CFCF. The contents were gradually decreased with increased incubation. Maximum reduction was recorded in shoots treated with CFCF obtained from 14 days old fermented broth at 50% concentration followed by 75% and 100 % (Table 4). 14 days old fermented broth showed the maximum biological activity as depicted by the significant reduction in the chlorophyll and protein content of the host leaves. While extract obtained from 7 days old broth failed to show any remarkable reduction in these contents at similar concentration. The effect was comparatively more on chlorophyll-a and total chlorophyll while chlorophyll-b and protein contents were less affected.