Arthrobacter crystallopoietes growing exponentially as cocci were changed to rods by adding succinate to the medium. Cells were sampled before, during, and after this transition for Gram-staining and ultrastructural studies. Cells were Gram stained by the standardized method of Bartholomew, and all samples were fixed and prepared for thin sectioning in an identical manner. Cocci were gram positive, and thin sections demonstrated a gram-positive type of cell wall having an average thickness of 31 nm. Cells sampled during morphogenesis appeared as cocci with most having a single rodlike projection. The coccus portion of these transition cells was gram positive and bound by a gram-positive type of wall having an average thickness of 29 nm. The rodlike projection of the transition cells appeared to be gram negative; it was also surrounded by a gram-positive type of wall, but its average thickness was only 22 nm. Gram-negative rods of the type species, Arthrobacter globiformis, were also examined and found to produce a gram-positive type of wall with a 19-nm average thickness. Evidence for the trilaminar region, characteristic of most gram-negative bacterial cell walls, was totally lacking in both species. These results suggest that variations in cell wall thickness may be an important contributing factor to the variable Gram-staining characteristics of this genus.