Species of theParamecium aureliacomplex, characterized by inbreeding, show differences in the number of chromosomes, not only between species, but also within species. It seemed interesting to study the karyologic differentiation inParamecium jenningsi, a taxonomic species of the genusParameciummorphologically similar to theP. aureliaspecies complex, but characterized by outbreeding. Applying the method of semi-thin sections it was found thatP. jenninpistrains from India, Uganda, and Madagascar differ karyologically from each other.