Changes in gene expression observed in vivo in response to desiccation and rehydration of the desiccation-tolerant bryophyte Tortula ruralis are regulated by alterations in the pattern of selection of messenger RNAs, from a qualitatively constant mRNA pool, by the translational machinery. When drying rates are slow, messenger ribonucleoprotein particles (mRNPs) are formed in the drying gametophytes. A representative rehydrin mRNA, Tr288, was sequestered into these particles which were analysed using sucrose and CsCl gradients. Quantitative RT–PCR analysis of the fractions from a low salt extraction demonstrated that Tr288 mRNA migrated farther in the sucrose gradient, relative to those extractable in high salt, indicating that the transcript is associated with particles that are of higher density. RT–PCR analysis also demonstrated that the majority of Tr288 mRNA, from slowly desiccated gametophytes, is associated with particles that have buoyant densities between 1.44 and 1.64 g cm– 3 which correspond to the buoyant density range reported for mRNP particles. mRNPs that are unique to drying T. ruralis gametophytes form at least four size classes after in vivo UV cross-linking based upon FPLC analysis. This is the first report of mRNP formation in response to a vegetative water deficit in plants.