Abscisic acid (ABA) induces formation of a set of proteins in the xerophilic liverwortExormotheca holstii. Some of them have immunological properties similar to the dehydrins of desiccated corn embryos and the desiccation-related proteins of Craterostigma plantagineum. The fluctuations of endogenous ABA during cycles of desiccation and rehydration seem to be sufficiently high to indicate a role for ABA as a stress hormone and there by as an endogenous inductor of stress-related protein synthesis. Desiccation tolerance disappears when thalli are cultivated for a longer period under well-watered conditions; such thalli are not able to increase stress-dependent ABA biosynthesis sufficiently, or to form the desiccation-related proteins unless they are treated with external ABA. The rehydrated thalli cannot recover from a rapid water loss, while ABA-treated, non-hardened thalli regain their photosynthetic activity within two hours. Keywords: Abscisic acid, Desiccation tolerance, Desiccation-related proteins, Exormotheca, Marchantiales.