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Some alien tree species used in commercial forestry cause major problems as invaders of natural ecosystems. One such case, the black wattle tree, was introduced into South Africa from Australia in the 19th century. It is an important commercial species, as well as an aggressive invader, giving rise to significant environmental impacts and conflicts of interest. This paper provides an analysis of costs and benefits associated with this species in South Africa at a national level. The results suggest that a ‘do nothing’ scenario (with no attempts being made to control the spread of the species beyond the limits of plantations) is not sustainable, as the benefit–cost ratio is around 0.4. The most attractive control option will be to combine physical clearing and plant-attacking biological control with the continuation of the commercial growing activities. In case this is not practically feasible the next best option is a combination of seed-attacking biological control, physical control and the development of secondary industries based on wood products from clearing programmes. There is, however, a 40% loss of benefits involved with this option when compared with the first best option. The techniques used in this study, and the findings relating to the scenarios that deliver the best returns on investment, should be of broad relevance to the problem of dealing with conflicts of interest relating to invasive alien plants that have commercial value. [ABSTRACT FROM AUTHOR]