Spiroplasma sp. strain SMAT, isolated in Poland from white satin moth larvae, Leucoma salicis L. (Lepidoptera: Lymantriidae), was serologically distinct from other Spiroplasma species, groups or subgroups. Dark-field microscopy of the cells revealed the classical helical shape and subsequent transmission electron microscopy revealed cells surrounded by only a single cell membrane (lacking a cell wall). Growth of strain SMAT occurred in M1D medium at 30 °C. Strain SMAT catabolized both glucose and arginine, but did not hydrolyse urea. The G+C content of the DNA was 24±1 mol% as determined by melting temperature analysis. Serological analysis revealed a very weak cross-reactivity (positive reaction only up to a 1 : 80 dilution) with two Spiroplasma strains, 277F (Spiroplasma sp. group I-4) and LB-12 (Spiroplasma sp. group I-5). Strain SMAT (=ATCC BAA-521T=NBRC 100392T) is designated the type strain of a novel species, Spiroplasma leucomae sp. nov. (class Mollicutes: order Entomoplasmatales: family Spiroplasmataceae).