Ultrastructure of Rickettsiella phytoseiuli (R.p.) multiplying in female ticks Dermacentor reticulatus was compared with that of Coxiella burnetii (C.b.) in the same ticks and in mice. C.b. in ticks and mice were always represented by 2 main cell types: small dense round or rod-like cells (DC) and larger bacteria-like cells (BC). DC were surrounded with a characteristic five-layered 20 nm thick envelope. Under the envelope DC had a stack of parallel intracytoplasmic membranes with a periodicity 5-6 nm. R.p. in tick fat body and synganglion were also inside phagosomes and formed 6 sequentially developing cell forms: dense (elementary), intermediate, bacterial, giant, and crystal-forming in which small dark bodies (initial particles) condensed. Two of them--dense and bacterial--corresponded to DC and BC of C.b. The DC envelope structure of R.p. was strikingly similar to that of some C.b. DC in mouse. We confirmed the general morphologic similarities in the structure of C.b. and R.p. DC and that of C.b. BC and intermediate cells of R.p. The envelope structure of DC type was found in other gracilicute bacteria and is supposed to have no taxonomic value but to be a reflection of population heterogeneity.