Spiroplasmas are helical mycoplasmas which are found in plants and arthropods, also in ticks. Some Spiroplasma species are incriminated as potential pathogens for vertebrates. During a study on Q fever in North Rhine-Westphalia, an intracellularly growing microorganism could be isolated from a pool of Ixodes ticks. The agent replicated within cytoplasmic vacuoles similar to those of Coxiella burnetii. PCR using coxiellae-, ehrlichiae- or chlamydiae-specific primers showed that agent Z/16 was distinct from these bacteria. In contrast to coxiellae or chlamydiae, the agent could not be stained according to the method of Giménez. Also electron microscopy provided evidence that the isolate Z/16 is different from coxiellae and chlamydiae. Determination of 16S ribosomal RNA gene sequences provided evidence that the isolate Z/16 can be classified as Spiroplasma sp. To our knowledge, this is the first report of an isolation of a Spiroplasma strain using a mammalian cell line. The pathogenic potential of the organism needs further investigation.