Survey and field experiments were conducted in the Department of Agronomy, Annamalai University during 2000 and 2001 to study some of the ecological traits of Congress grass (Parthenium hysterophorus L.) in Veeranum Ayacut region of Tamil Nadu state and to compare the bio-efficacy of some herbicides for controlling this weed. Results showed that the weed had two generations in a calendar year. Congress grass germinated with maximum temperatures between 30° and 34° C coupled with available soil moisture between 40 and 60% coincided with February and September months. The weed occurred only in the wastelands and did not expand into cultivated fields. Associated flora included predominantly grasses. Glufosinate ammonium @ 2.5, 3.0 l/ha and glyphosate @ 2.5 l/ha were effective in imparting a complete control of Congress grass