Facultative bacterial endocytobionts of the perinuclear space in Ciliophora, Chrysophyta and Kinetoplastida (14 species in all) are described using the authors' data and excerpts from the literature. A morphometrical analysis of the bacteria indicates that they are commonly Gram-negative, rod-like particles with similar dimensions and morphology. The bacteria of the nuclear envelope of paramecia do not have infectious capacity or killer traits. The killing capacity and infectivity of other perinuclear endocytobionts have not yet been determined. In light of the similar morphology of the bacteria and their location in the host cell it is possible that most of the endocytobionts are from a single phyletic group. The types of endocytobiosis are considered with respect to the evolution of the bacterium-host interaction.