The aim was to improve the rumen degradability of wheat straw (WS), which has relatively high cell wall content. Plant cell walls consist of hemicellulose and cellulose that are bound to lignin. These carbohydrates can be an important source of energy for rumen microbes. However, rumen microbes cannot degrade lignin, which blocks the availability of the carbohydrates. The availability of carbohydrates can be increased when lignin is removed in a pre-treatment. In nature, dead plants can be degraded by fungi. Some fungal species degrade lignin without consuming cellulose during vegetative growth. One of the selective lignin degrading fungi, Lentinula edodes was used to test the improvement in rumen degradability of WS. Two conditions were tested in triplicate: autoclaved WS inoculated with L. edodes and autoclaved WS as control. After 12 weeks of incubation at 24°C, rumen degradability was determined with the in vitro gas production (IVGP) technique (Cone et al., 1996). Lignin, hemicellulose and cellulose content were determined according to the methods described by Van Soest et al. (1991). Changes in chemical composition and IVGP upon fungal treatment were compared with the control, using the generalized linear model method in SAS (v9.3). To test the effect of changes in lignin structure and properties, pyrolysis gas chromatography-mass spectrometry (py-GC/MS) was done on fungal treated WS and the control. L. edodes treatment for 12 weeks increased (P < 0.05) IVGP of WS compared with untreated WS. Cellulose content was unchanged, while hemicellulose and lignin content decreased (P < 0.05). In addition to a decrease in total lignin, py-GC/MS showed an increasing amount of lignin degradation products. Upon L. edodes treatment not only a total degradation of lignin occurred, but the composition of lignin also changed. Lignin in WS consists of syringyl (S) and guaiacyl (G) units in a 1:1 ratio. L. edodes degraded more S than G units, since the S/G ratio decreased. This decrease in S/G ratio was correlated to an increase in IVGP. We conclude that the L. edodes treatment increased the IVGP of WS, which was correlated to both lignin content and composition.