The effect of O-methyltransferase (OMT) cDNA modulation on cell wall composition, ultrastructure and rumen degradability was measured on transgenic tobacco (Nicotiana tabacum). The expression of OMT cDNA in antisense orientation (AS) inhibited OMT activity by 92% whereas expression of sense constructs led to plants either co-suppressed (CS, 98% inhibition) or overexpressing OMT activity. The cell wall residues of stems were analysed for lignin content, products of nitrobenzene oxidation (NBO) and polysaccharide content. Degradability was determined by a cellulase method. Sections of stem were stained by acid phloroglucinol and Mäule reactant. Stem samples were incubated in the rumen for 8, 24 and 48 h and observed by scanning electron microscopy (SEM). Compared to controls, OMT-depleted stems showed decreased hemicellulose content but unchanged lignin content. In contrast, syringyl units decreased by 40 and 90% in AS and CS samples respectively and NBO content followed a similar trend. Dry matter cellulase degradability was significantly improved by 3·5 and 5·6 percentage units in AS and CS samples respectively. SEM showed a greater bacterial colonisation in these samples and indicated a higher rate of rumen degradability in CS tissues than in controls. Overexpressing plants had a composition and a degradability similar to that of controls. For all the plants studied, the improvements in dry matter degradability were closely linked to the syringyl to guaiacyl ratio or to the NBO content. The modifications observed in down-regulated tobacco were similar to those produced by bm3 maize mutation, but without lignin decrease. Genetic modifications should therefore be considered for improving forage digestibility.