Many of New Zealand’s endemic non migratory freshwater fish species are currently threatened as a result of competitive exclusion and/or direct predation by salmonids. Tools to eradicate and manage exotic fish species in running waters in New Zealand are presently limited. Interestingly, salmonids exhibit high sensitivity to low pH water relative to many native New Zealand species. This vulnerable aspect of their physiology was used to explore the possibility of using low pH to 'repel' trout in static and flowing waters. Laboratory experiments were conducted to test the response of trout to three pH levels; pH 7.0 (neutral control), pH 5.5 and pH 3.5. Four replicate trials of each were conducted with 'naive' brown trout (80-150mm) being used for each trial. Responses of trout to the same pH levels were investigated in static and flowing choice chamber units. Responses of trout pre and post treatment were analysed by viewing taped video recordings of fish movement between chambers during the pre and post observation periods. Trials indicate significant potential for further development of this tool with a very strong avoidance response by trout to both the mildly acidic (5.5) and strongly acidic (3.5) levels relative to control trials. No mortality of trout was seen and the use of acidification may be preferred by other stakeholders if trout can be removed without killing. The small quantity of acid required per volume of water to attain a desired avoidance response means that the logistics of using this tool in small streams without major adverse effects is a real possibility.