Phytotoxic metabolite produced by the five different isolates of Sarocladium oryzae, the rice sheath rot pathogen, was isolated from its in vitro culture filtrate to know the variability among isolates in terms of toxin production. The toxin was purified through solvent extraction and then by gel filtration on sephadex column. Carbohydrate and protein content of the toxin and the quantity of phytotoxin produced in planta were varied among isolates. Toxin from the five isolates also showed variation in its biological activity as detected in its ability to induce sheath rot symptoms on detached leaf sheath, greenhouse grown plants and to cause leakage of electrolytes from rice leaf sheath. The symptoms produced by the purified toxin were, symptoms similar to those of natural infection by the pathogen. The virulent isolates SO1 and SO2 produced more amount of toxin, more leakage of electrolytes and severe sheath rot symptoms. The in planta toxin production by different isolates was detected serologically, using the polyclonal antibody produced against SO1 toxin. Correlation between the variation in toxin production and the virulence of the pathogen was discussed. © 2007 Asian Network for Scientific Information.