Three cows fitted with large rumen fistulas were used in investigations of relationships between growth of microbial populations and activity of fibrolytic enzymes in relation to diet. The diet consisted of barley grain and hay and was supplemented with rapeseed meal or fish meal. The total number of ciliates varied from 121.3x10³ and 223.2x10³/g of rumen contents and was significantly higher when the feed was supplemented with fish meal. Bacterial mass was 11.0-14.9 mg/g rumen contents irrespective of diet. The determinations were based on the DAPA concentration in bacteria and in rumen digesta. The biomass of protozoa in the whole rumen varied from 208.2 to 358.6 g, that of bacteria ranged from 986.3 to 1391.6 g. Fish meal supplement increased the biomass of protozoa before the morning meal and 4 h thereafter and the biomass of bacteria before feeding. Xylanase activity in whole rumen digesta varied from 68.4 to 96 µmol reducing sugars released from xyian/g DM/min and was higher when the feed was supplemented with rapeseed meal. The activity of CMC-ase was 6.2-6.9 µmol reducing sugars released from CMC/g DM/min. No relationship was found between either enzyme activity and total bacterial mass in the rumen contents The xylanase and CMC-ase activities associated with particulate fractions varied from 34.0 to 63.3 and from 1.9 to 4.6 µmol of reducing sugars released from xylan or CMC respectively. The activity was higher when cows were fed diet supplemented with fish meal, and this was accompanied by higher counts of protozoa. A negative correlation was found between the activity of CMC-ase associated with particulate fractions and ADL content in this material.