Inhibitory effect of Cassia occidentalis L. (CO) on Parthenium hysterophorus L. (PH) was assessed under various biochemical parameters like chlorophyll, nitrogen and protein percentage. Chlorophyll content, nitrogen and protein percentage of one month old Parthenium plants were observed after treatment with aqueous leachates of the shoots of C. occidentalis and compared with control sets treated with distilled water. The allelochemicals released from Cassia inhibited the chlorophyll, nitrogen and percentage of Parthenium to the tune of 84.9, 96.4 and 96.4%, respectively. The results indicated the possible suppressive effect of allelochemicals present in C. occidentalis L. (CO) on P. hysterophorus.