Weeds are responsible for extensive annual losses of agricultural production although convincing estimates of such loss are largely unavailable, particularly in Australia. Recognition of the economic cost of weeds is important for a number of reasons which include the rationalization of weed control programmes and the direction of weeds research. Because of data deficiencies relating to extent and distribution, impact on production and the environment, and the spread potential of individual weed species, this area of weeds research has to date received little attention. In this paper, the concepts of economic surplus are utilized in a general social benefit framework to assess the economic importance of serrated tussock (Nassella trichotoma), the major pasture weed of the New South Wales tablelands. The widespread control of serrated tussock under pasture improvement would result in substantial increases in social benefits at both the state and national levels. Some of the important questions surrounding the social benefit model and its )practical application are also discussed.