The rumen bacterium Butyrivibrio fibrisolvens has been genetically modified to detoxify fluoroacetate (a poisonous component of trees and shrubs in Australia, Africa and Central America) and has been shown to persist when it is returned to the rumen. Such bacteria may save animals from poisoning and, therefore, reduce economic losses for livestock industries in those countries. The ability to make genetic changes to rumen bacteria raises important questions about their practicality, and about the environmental factors that must be considered before releasing modified strains. The fluoroacetate-detoxifying bacterium provides an important model by which these issues can be examined.