In this paper we report our ultrastructural studies of the early phagocytosis of two different strains of L. pneumophila serogroup (SG) 1 (Philadelphia 1 and Knoxville 1) and of L. micdadei. These bacteria replicate, in vivo as well as in vitro, in eukaryotic cells e.g. in monocytes and macrophages. Whether or not the mode of entry of these organisms in phagocytes contributes to their intracellular survival is presently unknown. Whilst internalization of bacteria of the Philadelphia 1 strain occurred within a pseudopod coil, organisms of the Knoxville 1 strain and L. micdadei were phagocytized in the classical manner, i.e. between pseudopods. No ultrastructural differences were observed between the two strains of L. pneumophila SG 1 whereas L. micdadei appeared as shorter rods with an extracellular layer of relatively low electron density. The phenomenon of coiling phagocytosis was not affected by heat-killing the bacteria or preopsonization with specific antibody. Formation of phagolysosomes was seen when cells of the Knoxville strain and L. micdadei were used but not with the Philadelphia strain. In our experiments, the occurrence of coiling phagocytosis was specific for the Philadelphia 1 strain of L. pneumophila and independent of bacterial virulence. Thus, it seems most unlikely that the coiling phenomenon plays any important role in the resistance of Legionella to the killing abilities of phagocytic cells.