This review describes the pathway a plant pathogenic mycoplasma or spiroplasma takes in its passage through a leafhopper vector. Reference is made to several strains of spiroplasma and acholeplasma, but, in particular, data are presented for Spiroplasma citri and the corn stunt spiroplasma. Acquisition of the organisms is discussed, together with the different methods of infection (feeding on plants and through membranes or following injection) and the effect they have on the inoculum dose. The dose, together with the environmental conditions, are also factors which effect multiplication in both whole insects and salivary glands. Titers reached by the organisms in the insect are given. Pathogenic effects on the insects are discussed. The analogy is given of the insect acting as a chemostat with poor nutrition or high temperature adversely affecting the balance. Feeding behavior and the number of organisms ejected are two factors affecting transmission.