Virus replication in plant, animal, or human tissues is a process that requires cellular machinery. Viruses consist of a nucleic acid genome capable of self replication and coding for a variety of proteins. Mycoplasmas, whether intracellular or extracellular in their habitat, are nearly complete "cells" in their own right. This chapter summarizes the research reported on the structural, biochemical, and immunological characteristics of plant pathogenic spiroplasmas and mycoplasma-like organisms (MLOs). The first sustained cultivation of a spiroplasma was achieved by French workers in a relatively simple medium, with a composition similar to that used for the growth of animal mycoplasmas. Helical morphology of the spiroplasmas was first revealed by phase contrast light microscopy of the sap from plants affected with corn stunt disease. The spiroplasmas and all other mollicutes were found to carry one or two rRNA operons as opposed to several repeats of rRNA operons present in the eubacterial chromosome.