Enchytraeidae is one of most insufficiently studied Baikal oligochaete families. Mesenchytraeus bungei Michaelsen, 1901, an endemic representative of this family, often dominates in abundance and biomass in the splash zone. The contribution of Enchytraeidae in the total number of oligochaetes grew from the water edge to the slope. Oligochaete numbers increased from May until July and droped from August to October. Their abundance peaked in late July–early August. The average number is 47 139 (24 829–79 525) specimens/m2. A qualitative study of the seasonal dynamics of the food spectrum of M. bungei was performed for the first time. In summer, the ration mainly included plant detritus, while in autumn M. bungei fed on both plant and animal detritus. In spring, diatoms made up the basis of M. bungei diet. The species prefers green filamentous alga Ulothrix zonata (dominating at the water edge) which is most common in June, a period when the lake water level rapidly increases and shore detritus is formed from dead U. zonata. Preliminary data obtained show that zoospores of U. zonata remain viable after passing through the intestines of Enchytraeidae.