X-ray microdensitometric analysis was employed for the detection of fungi attack in wood of pine and beech in comparison with the studies of specimen weight loss tests. Two species of fungi were used: Gloeophyllum trabeum (Pers.:Fr. Murrill) and Trametes versicolor (L.:FU. Pilat). We select these species to induce typical decay attack mainly on cellulose in pine (brown rot) and mainly on lignin in beech (white rot). The attack was conducted for 1, 2, 3, 4 and 5 months in sterile laboratory conditions. After 5 months all the values of density components decreased. In beech the loss was of about 25% for all components. In pine the decreasing of the earlywood density component was of about 10% and in latewood of about 22%. The corresponding mass density losses, determined by gravimetric method, were approximately 18% for beech and 16% for pine.