A filter paper spot test with 2'-(4-methylumbelliferyl)-alpha-D-N-acetylneuraminic acid as a substrate was used to study the prevalence of sialidase activity among gram-negative anaerobic and capnophilic bacteria. A total of 567 isolates representing four genera of obligate anaerobes and four genera of capnophilic organisms was tested. Sialidase activity was detected in 94% of 66 isolates from the Bacteroides fragilis group, 98% of 66 B. bivius isolates, and all isolates of the following species (number of isolates follows species name): B. capillosus, 4; B. levii, 2; B. denticola, 22; B. loescheii, 23; B. melaninogenicus, 32; B. forsythus, 44; and B. buccalis, 2. However, sialidase activity was detected in only 29% of 7 B. buccae isolates, 79% of 14 B. disiens isolates, and 55% of 11 B. oralis isolates. Sialidase activity was not detected among any of 13 isolates of B. gracilis, 12 isolates of B. ureolyticus, 61 isolates of B. intermedius, or 26 isolates of B. corporis. Porphyromonas (Bacteroides) asaccharolytica (20 isolates) and P. endodontalis (8 isolates) did not demonstrate sialidase activity, while 25 isolates of P. gingivalis were sialidase positive. Sialidase activity was found in 10 (100%) of 10 isolates of Capnocytophaga ochracea of C. sputigena but not in any of 4 C. gingivalis isolates. Other gram-negative anaerobic or capnophilic bacteria, including the following, were negative for sialidase activity: Fusobacterium nucleatum, 39 isolates; Wolinella recta, 19 isolates; Eikenella corrodens, 17 isolates; Haemophilus aphrophilus, 10 isolates; and Actinobacillus actinomycetemcomitans, 10 isolates. These data demonstrate sialidase activity in several species of the genera Bacteroides and Porphyromonas and suggest that this characteristic may be useful for identification.