Abstract: S temp hy lium botry osum could p roduce patho toxin in v itro. F ive fract ions (Ⅰ、Ⅱ、Ⅲ、Ⅳ、Ⅴ ) had been obtained by TLC and HPLC ch romatography, their Rf-values w ere 0.31, 0.47, 0.74, 0.80 and 0.90,and their max. ult ravio let abso rp t ion w ere 240, 247, 223, 236 and 258nm , respectively. Three fractions (Ⅱ、Ⅲ、Ⅳ ) st rongly inh ibited the growth of Digitaria sanguinalis, among them the fraction Ⅳ had the highest activity. The inh ibito ry effect of crude toxin on the germination of gramineous weed seeds was much higher than that of the broad-leaf weeds. This toxin has little influence on the seeds ofmaize, rice, rape and Chinese cabbage. The results of the greenhouse potexperiment indicated that the defensive effect of the crude toxin on Digitaria sanguinalis was the highest, equal to the effect of paraguar. Mo reover the toxin could affect the content of Chlorophyll in Digitaria sanguinalis; and especially the content of Chlorophyll-b.