The following strains: B. fragilis subspecies thetaiotaomicron (A); B. fragilis subspecies fragilis strain E-1, E-2, M, St., Se., Ni., 8, 16; B. fragilis subspecies distasonis 145 (D) were serologically studied by immunofluorescence as compared to agglutination. Anti-B. fragilis sera titration by immunofluorescence (IF) reaction, as compared to agglutination reaction in tube, was more sensitive (2-16 times higher titers), specific and reproducible. Among the organisms from B. fragilis group, species, subspecies and even train specificity was noticed. Also, the possibility for rapid identification of anaerobic organisms, belonging to B. fragilis group, in pathologic products obtained from experimentally infected animals (mice and rats), by IF reaction, in comparison with classic methods (anaerobic cultures and biochemical determinations) was studied. Of 87 studied animals, 61 proved positive by cultures and 59 by IF; 56 animals were shown positive and 23 animals proved negative by both methods (intermethods concordance in 79 cases). Statistical analysis of IF results provided the following: method sensitivity (detection capacity of real-positive cases)-91.80%; method specificity (detection capacity of real-negative cases)-88.46%; false-positive cases-11.53%; false-negative cases-8.19%. Immunofluorescence proved specific, sensitive, practical and rapid method detection of non-sporulated anaerobic organisms species and subspecies belonging to Bacteroides fragilis group.