A technique is described for rearing germ-free desert locusts. Axenic insects appeared to develop and reproduce normally, therefore it is concluded that the gut bacterial flora does not contribute significantly to locust nutrition. However, it is suggested that the gut flora may benefit its host by contributing to the locust's defences against attack by pathogenic microorganisms. The results of morphometric measurements are discussed in terms of the bacterial origin of the so called phase pheromone, locustol. No evidence was found to support the hypothesis that microbial activity restricts gut carbohydrase.