Résumé : In a case of gallbladder infection, bacteriological studies revealed the presence of Actinomyces israelii, an organism rarely encountered in this site. While direct or indirect immunofluorescence techniques would undoubtedly provide the best identification criteria in the shortest time, the antisera were not available when the study was conducted. The most rapid technique for a tentative identification of isolated colonies of the strain was based upon detection of glycosidases together with catalase activity. Although not sufficient since A. israelii and A. naeslundii present the same enzymatic profiles, the other species and the most closely related genera can nevertheless be excluded by this method. Gas liquid chromatography - indeed useful yet much slower - contributes to identification but is not suitable for differentiation at the species level. Accurate identification was obtained by classical, time-consuming techniques. Distinguishing A. israelii from A. naeslundii ten days following pathogen isolation can only be of limited practical interest. © 1981 MMW Medizin Verlag.