Morphological studies of the hepatopancreas of the terrestrial isopod Porcellio scaber revealed bacterial infection. The percentage of infected animals collected from the same site varied from 0 to 10% during the 4 years of study. Transmission electron microscopy, scanning electron microscopy, and light microscopy revealed that infected glands differed from those in healthy isopods. The most prominent sign was white spots between 100 and 200 &mgr;m in diameter along the entire gland. These spots were aggregations of vacuoles in the cells that were densely filled with bacteria in different phases of the developmental cycle that included the formation of small, dense, rod-shaped infective bacteria and much larger spherical multiplying cells filled with aggregates of polysomes and a chromatin network. Occasionally, large sphericles were filled with homogeneous electron-dense material. Bacteria were not observed in the cell nucleus. Small vacuoles of less than 5 &mgr;m were filled predominately with spherical bacteria but rod-shaped forms were also present in large numbers. Larger vacuoles of 10 to 20 &mgr;m in the main were densely filled with rod-shaped bacteria. According to the literature on the morphological characteristics of bacteria infecting invertebrates, those described in our study would be classified in the genus Rickettsiella. However, most recent investigations show that besides morphological investigations, genetic ones are also needed to define the taxonomic position of bacteria that infect invertebrates. Copyright 1999 Academic Press.