Over the course of several million years, the eukaryotic gut symbionts of lower termites have become adapted to a cellulolytic environment. Up to now it has been believed that they produce nutriments using their own cellulolytic enzymes for the benefit of their termite host. However, we have now isolated two endoglucanases with similar apparent molecular masses of approximately 36 kDa from the not yet culturable symbiotic Archaezoa living in the hindgut of the most primitive Australian termite, Mastotermes darwiniensis. The N-terminal sequences of these cellulases exhibited significant homology to cellulases of termite origin, which belong to glycosyl hydrolase family 9. The corresponding genes were detected not in the mRNA pool of the flagellates but in the salivary glands of M. darwiniensis. This showed that cellulases isolated from the flagellate cells originated from the termite host. By use of a PCR-based approach, DNAs encoding cellulases belonging to glycosyl hydrolase family 45 were obtained from micromanipulated nuclei of the flagellates Koruga bonita and Deltotrichonympha nana. These results indicated that the intestinal flagellates of M. darwiniensis take up the termite's cellulases from gut contents. K. bonita and D. nana possess at least their own endoglucanase genes, which are still expressed, but without significant enzyme activity in the nutritive vacuole. These findings give the impression that the gut Archaezoa are heading toward a secondary loss of their own endoglucanases and that they use exclusively termite cellulases.