A total of twenty four French strains and two reference strains IPO323 and IPO94269 of the hemibiotrophic fungus Mycosphoerella graminicola were investigated in planta to examine the association of the cell-wall degrading enzyme endo-beta-1,4-xylanase (EC 3.2.1.8) with pathogenicity. The French strains were selected from a collection of 363 strains previously genotyped using microsatellites, actine and beta-tubutine markers. Disease level assessments as well as enzyme quantifications were carried out at 20 days post inoculation from the third leaves of inoculated whole plants of the susceptible wheat cv. Scorpion. Great variability of both pathogenicity levels and endo-beta-1,4-xylanase activity patterns was obtained among strains. Only 15 out of the 26 assessed strains including the reference strain IPO323 were able to induce lesions bearing pycnidia. The percentages of diseased leaf areas bearing pycnidia ranged from 6.2% to 77%, while amounts of endo-beta-1,4-xylanase activity ranged from 0 to 399.15 mU/microg of total proteins. A Pearson correlation test revealed very high linkage between endo-beta-1,4-xylanase activity level and lesions bearing pycnidia production within strains (r = 0.94). Additional cytological and enzymatic investigations on two strains exhibiting different pathogenicity levels highlighted that successful disease induction by M. graminicola is not explained by either spore germination or direct and stomatal penetration rates of the host, but by the ability of the fungus to colonize the mesophyll and to secrete the endo-beta-1,4-xylanase activity during necrotrophic phase. This study strongly suggests the importance of both mesophyll colonisation and endo-beta-1,4-xylanase activity during the infection process of M. graminicola.