Some paramecia express the so-called killer trait: killer paramecia release toxic particles into the environment and these particles kill sensitive Paramecium cells after ingestion. These particles are bacteria belonging to the genus Caedibacter, endosymbionts of killer Paramecium strains. A peculiarity of Caedibacter is the presence of an unusual structure termed R-body (refractile body) due to its bright appearance in phase contrast microscopy. It is constituted of a proteinaceous ribbon which is typically coiled within the bacterial cell. In response to certain stimuli, e. g. a low pH, the R-body unrolls in a telescopic fashion and thereby produces a hollow tube, several times the length of the bacterium which is destroyed by the extension. R-bodies are prerequisite for the killer trait. Their structure and mode of action suggest that they act as a releasing system for an unidentified lethal toxin by delivering it from the bacterial cell to the cytoplasm of the sensitive Paramecium where it finally causes death. The expression of the killer trait confers a competitive advantage to Caedibacter-harbouring paramecia, but it also inflicts costs on both partners of this symbiotic association.