Straight or slightly curved rods, 0.8-0.9 by 1.5-8 um, occurring singly , or occasionally in pairs in tandem in the host insect. The longer forms occur in mycetocytes in the abdominal fat body , the shortes forms in the germ tissue and oocytes. Microorganisms in mycetocytes are encapsulated by a host – provided membrane which is continuous with host – cell organelles. Non – motile. No resting stages known. Gram – positive, becoming indeterminate (speckled, barred or negative) in malnourished hosts. Cell wall 5 – 10 nm composed of several alternating electron opaque and transparent layers. Muramic acid and glucosamine present. Complex fibrillar mesosomes neas site of formation of cross-walls.