The invasive herbaceous species Parthenium hysterophorus L. (Asteraceae), commonly known as parthenium weed has rapidly become a significant weed in more than 30 countries. Parthenium weed litter taken from the introduced biotypes was relatively more phytotoxic than that taken from biotypes coming from the native range when tested on lettuce and this may indicate one reason for invasion success. However, no significant difference was observed in phytotoxicity to lettuce seedling growth when two Australian biotypes of parthenium weed were compared, one invasive and one non-invasive, indicating that invasiveness was not associated with litter phytotoxicity in all cases. Residue from the invasive parthenium weed biotype had a greater phytotoxic effect upon Australian native pasture grass species relative to the introduced pasture grass species with buffel grass (Cenchrus ciliaris L.) and bull Mitchell grass (Astreble sequarrosa C.E.Hubb) showing the greatest tolerance to parthenium weed phytochemicals. When compared with residue taken from plants that has a range of phytotoxic capacity, parthenium weed residue was considered to be only moderately phytotoxic suggesting that the phytotoxicity of its residue may not be the main reason for the plants invasive trait.