Nine strains of spiroplasma subgroup 1-3, which comprise the etiological agent of corn stunt disease, were similar in their serological properties. Strain E275T (T = type strain) was studied by using criteria proposed by the International Committee on Systematic Bacteriology Subcommittee on Taxonomy of Mollicutes for descriptions of new mollicute species. This strain was shown to belong to the class Mollicutes by the ultrastructure of its limiting membrane, its procaryotic organization, its colonial morphology, and its filtration behavior and to the family Spiroplasmataceue by its helical morphology and motility. Although some serological cross-reactions with other group I spiroplasma strains was observed, strain E275T could be readily distinguished from representatives of other group I subgroups. Subgroup 1-3 spiroplasmas and other group I strains also differed in their one-and two-dimensional polyacrylamide gel protein patterns, plant and insect host ranges, and pathogenicities. Growth in M1A or M1D medium occurred at 20 to 3OOC. Cholesterol was required for growth. Glucose was fermented, and arginine was catabolized. Subgroup 1-3 strains, including strain E275T, reacted with considerable homogeneity in deformation tests and were completely separable from strains belonging to subgroup 1-1 (Spiroplasma citri) and subgroup 1-2 (Spiroplusmu melliferum). Strain E275T was also serologically distinct from subgroups 1-4 through 1-8, Spiroplasmu floricolu (group 111), Spiroplusmu apis (group IV), Spiroplusmu mirum (group V), and representative strains of spiroplasma groups I1 and VI through XI. The deoxyribonucleic acid of strain E275T hybridized with the deoxyribonucleic acid of S . citri at significant levels (33 to 68%, depending on the technique used). These results demonstrate that strain E275T and similar strains meet the criteria proposed by the International Committee on Systematic Bacteriology Subcommittee for elevation of spiroplasma subgroups to species. We propose that such strains be named Spiroplasma kunkelii. Strain E275T has been deposited in the American Type Culture Collection as strain ATCC 29320T.Corn stunt disease was first reported as a stunting and striping syndrome of maize in Texas (2). Although this disease was studied biologically (53,54,65,66), the procaryotic nature of the etiological agent remained unsuspected until the late 1960s. Then, prompted by reports that certain leafhopper-borne plant diseases thought to be of viral etiology were in fact caused by wall-less procaryotes, R. R. Granados and colleagues demonstrated that wall-less procaryotes were associated with "Louisiana corn stunt disease" in plant and insect vectors (43,44). These organisms could also be demonstrated in negatively stained preparations of sap from diseased corn (43). Following these morphological studies, Chen and Granados (13) achieved long-term maintenance of the Rio Grande strain of the corn stunt agent in cell-free media and observed filaments similar to those seen in vivo. Subsequently, phase-contrast microsc...