The biochemical reactions, carbohydrate content, and 16S-rRNA sequences of Tatlockia (Legionella) maceachernii and Tatlockia micdadei strains were studied. Except for catalase activity, Tatlockia strains were relatively inert in the biochemical tests commonly used in clinical laboratories. Phenotypically, the two Tatlockia species could be distinguished from other legionellae by the presence of yersiniose A, by their inability to hydrolyze hippurate or starch, by the absence of colony or media fluorescence, and by the absence of distinct browning of tyrosine-containing medium. These two species differed from one another by the production of acetoin by T. micdadei but not by T. maceachernii. Gelatinase activity, which had been reported in T. maceachernii, was observed in only one of the four strains studied. The 16S-rRNA sequences and carbohydrate profiles of T. maceachernii and T. micdadei were essentially identical. In preparing the RNA for study, it was noted that the 23S rRNA was fragmented in all T. maceachernii strains tested, while the 23S rRNA of T. micdadei strains was intact. Among the legionellae studied, T. maceachernii was most closely related to T. micdadei.