The control of weeds provides agricultural, environmental and social benefits. The agricultural gains typically dominate, and many studies have estimated these values. The environmental gains rest on the protection of plant communities from weed invasions, but these benefits have proved hard to value doe to the lack of data from relevant market transactions. An estimate of these gains is now derived from a set of Australia-wide costs to control 35 major weeds affecting the agricultural and forestry industries. Control costs varied closely with the number of threatened special conservation areas, and with the number of threatened plant species, within the distribution of the weeds. Across all agricultural weeds, control costs increased by about $5800 per year for each additional threatened special conservation area, and across all forest weeds, control costs increased by about $800 per year for each additional threatened special conservation area. Across all agricultural and forest weeds, control costs increased by at least $65,000 per year for each additional threatened plant species. Following standard economic assumptions about market transactions, these estimates can be taken as the values of the annual environmental benefits to the community from protecting an extra special conservation area or an extra species - given the budgets available to the weeds managers. These environmental gains alone appear to cover a substantial proportion of the control costs. The marginal estimates of value are extended to cases where many further areas are protected, by introducing certain assumptions about the change in marginal utility, and by using ecological information to help choose between these assumptions. The application and qualifications of these values are discussed.