Serrated tussock (Nassella trichotoma) and Giant Parramatta Grass (Sporobolus fertilis) are among the most noxious weeds in Australia. Both cause problems in pasture and there are limited control measures relying heavily on the herbicide flupropanate. With the recent confirmation of flupropanate resistance in serrated tussock and the report of suspected flupropanate resistance in Giant Parramatta Grass (GPG), this option appeared to be under threat.The aims of this thesis were to determine the extent of flupropanate resistance in serrated tussock and GPG in Australia and to understand the genetics of flupropanate resistance in serrated tussock. This thesis also documents the GPG resistance to 2,2-DPA and investigates a fungal pathogen, Nigrospora oryzae, as a potential biocontrol agent for GPG. A local paddock survey determined the spread and extent of flupropanate resistance in serrated tussock within 5 km of the original resistant site. The pot-dose method of assessing resistance identified plants resistant to flupropanate up to 3.5 km from the original site found in Victoria. Seeds from these plants showed 0-100% resistance, with sensitive plants often having a low („T5%) level of resistant seed. These results indicate the movement of flupropanate resistance through seeds or pollen and shows that its spread occurred within one year of detection. A national mail survey confirmed the massive impacts of serrated tussock across Australia, with annual serrated tussock costs ranging from $15,000 to $16,000 per year per respondent. This survey also identified the widespread infestation of this weed in a variety of land use patterns, from pasture to native grasslands, and the decrease in the value of farmland as a result. Heritability studies using controlled breeding experiments indicated a strong involvement of a maternal component in the inheritance of flupropanate resistance in serrated tussock, with a minor proportion of resistance heritable through pollen. GPG plants and seedlings were tested for flupropanate and 2,2-DPA resistance.Seedlings tested for flupropanate resistance were highly resistant (tolerating 33-39 times more than sensitive biotypes). With 2,2-DPA, resistant GPG plants did not die even at 14 times the field rate and resistant seedlings also showed 5-6 times more resistance than the sensitive biotype. The study has confirmed that flupropanate and 2,2-DPA resistance now exists in GPG.The potential of Nigrospora oryzae, a pathogenic fungus, as a biocontrol agent for GPG was determined. Mature plants and seedlings of GPG were inoculated with conidia of N. oryzae using three treatments (run-off, crown, and spray). Inoculated plants were smaller, with greater proportions of dead leaves (70% with the run-off a nd crown treatments and 53% with the spray treatment) than the control plants. GPG seedlings inoculated with N. oryzae were stunted and showed greater proportions of necrotic leaves in all the treatments than the control. There is potential to develop N. oryzae as a mycoherbicide to control GPG and further testing is warranted.