Recent global spread of the freshwater alga Didymosphenia geminata (Didymo) has caused major biosecurity and freshwater management concerns. Didymo degrades river esthetics, alters ecosystems, negatively impacts recre- ation experiences and may require strict containment measures. This paper assesses the impact of Didymo on nonmarket values for recreational angling using a case study from New Zealand, where Didymo is extraordinarily prolific. Choice experiment data are used to fit a latent class model, identifying five distinct angler preference profiles. For the largest class of anglers the presence of Didymo had no significant effect on angling benefits. How- ever, other classes were negatively affected. Overall, Didymo reduced fishing values by about NZ$44 per visit. An- glers were sensitive to the scale of Didymo infestation, suffering significantly higher costs when more water bodies are affected. Closure of Didymo-infected mainstem-rivers to prevent spread of Didymo resulted in a sig- nificant reduction in angler net benefits, even if the policy were 100% effective. The latent class model identified distributional implications. While three angler classes would not have significantly different benefits if mainstem-rivers were closed to prevent the spread of Didymo, two classes of anglers would be highly impacted by such a policy, particularly those who regularly fish mainstem-rivers for salmon.