A new bacterial strain, designated SHRIMPT, isolated from the haemolymph of the Pacific white shrimp, Penaeus vannamei, was serologically distinct from other spiroplasmas. Cells of this strain were helical in form and variable in length. Examination by electron microscopy revealed wall-less cells delineated by a single cytoplasmic membrane. The organisms grew well in M1D media supplemented with 2 % NaCl. Strain SHRIMPT grew at temperatures of 20–37 °C, with optimum growth occurring at 28 °C. The strain catabolized glucose and hydrolysed arginine, but did not hydrolyse urea. The G+C content of the DNA was 29±1 mol%. Strain SHRIMPT (=ATCC BAA-1082T=CAIM 1252T) is designated the type strain of a novel species, Spiroplasma penaei sp. nov., which represents a new subgroup (I-9) of the group I spiroplasmas.