Summary. Biosynthesis of the defensive compound pederin by females in the genus Paederus appears to be associated with endosymbiotic microorganisms. In P. melanurus, which could be maintained in the laboratory for several generations, horizontal transmission occurs by ingestion of eggs inoculated with the specific endosymbionts. They are responsible for females’ pederin polymorphism. Heating or freezing eliminated or markedly reduced transmission. This indicates bacteria that are unable to produce endospores and lacking a cell wall. Influences other than pederin biosynthesis could not be observed.