Cultures ofAcholeplasma laidlawii were inoculated with Mycoplasmatales virus-laidlawii 1 (MV-L 1) and examined by light and electron microscopy.A. laidlawii colonies that appeared collapsed or “cratered” were seen by electron microscopy to contain numerous rod-shaped virus particles, about 86 nm long and 13 nm wide, whereas these were not seen in normal-looking colonies. InfectedA. laidlawii cells were often seen, in negative stain, to be associated with tubular filaments up to 1 μm long and 13–18 nm wide which were probably derived from the limiting membranes of the cells. In thin sections, much virus was seen attached to the outside of the cells and palisade-like arrays of virus particles were found within about 25% of the cells. Helical structures up to 160 nm long and about 12 nm wide were also seen within cells of the same preparations. They may have been the nucleocapsids of another virus unrelated to MV-L 1.