Fibre digestion in vivo was studied in eight species of arthropods. The highest degree of fibre digestion (61–65%) was measured in Schistocerca gregaria, Eurycanta calcarata, and larvae of Pachnoda marginata. This approximates the fibre digestion of mammalian hindgut fermenters. The presence of propionic acid, butyric acid, and i-valeric acid, in the different compartments of the intestinal tract of arthropods appeared to be a good indicator of microbial fermentation. From the results it becomes clear that different strategies can be followed for the degradation of fibres. Cellulolytic enzymes can be produced by symbiotic microorganisms. In P. marginata larvae, fibre digestion was the result of this microbial activity. On the other hand, S. gregaria and E. calcarata, which possess low numbers of bacteria in their intestinal tract, degrade fibres with the aid of endogenously produced enzymes. The origin of cellulolytic enzymes did not influence the efficiency of fibre digestion.