Strain Tab4cT, a helical prokaryote that was isolated from the body of a Haematopota sp. fly collected in Champchevrier, Indre-et-Loire, Touraine, France, was found to be a member of the class Mollicutes. The cells of strain Tab4cT were small, motile helices that were devoid of a cell wall. The organism passed through filters with mean pore diameters as small as 0.20 mm. Strain Tab4cT grew rapidly in liquid SP-4 medium at both 30 and 37 degrees C. The organism fermented glucose but did not hydrolyse arginine or urea, and did not require serum for growth. In preliminary electrophoretic analyses, the cell protein patterns of strain Tab4cT were distinct from those of 14 other spiroplasmas found in mosquitoes, deer flies and horse flies from Europe and the Far-East. In reciprocal metabolism inhibition and deformation serological tests, employing antigens and antisera representative of spiroplasma groups I-XXXIII (including all sub-groups), plus ungrouped strains BARC 1901 and BARC 2649, no serological relationship with Tab4cT was found. The G + C content of the DNA of strain Tab4cT was about 25 +/- 1 mol% and its genome size was 1.305 kbp. It is proposed that spiroplasma strain Tab4cT be assigned to group XVII (presently vacant) and that strain (ATCC 700271T) is the type strain of a new species, Spiroplasma turonicum.