We studied cell size and shape, nuclear changes, and the ciliary pattern during conjugation of Protospathidium serpens, using protargol impregnation and morphometry. Preliminary data were gathered from Epispathidium ascendens and Apertospathula armata. Conjugation of P. serpens is temporary, isogamic, and without preconjugation divisions. Pair formation is heteropolar, and the partners unite obliquely with the oral bulge. The body becomes smaller and broader during conjugation, but no basic changes occur in the ciliary pattern. Conjugation and nuclear reconstruction follow the usual mode of ciliates. However, some peculiarities occur: only two of the four synkaryon derivatives of the second synkaryon division enter the third division and generate four macronuclear anlagen, which fuse to a single, long macronucleus strand. During conjugation, E. ascendens unites obliquely as P. serpens, while A. armata can pair dorsal-to-dorsal surface, ventral-to-dorsal surface, or obliquely as P. serpens. The nuclear processes of these three species are also rather different, showing a considerable diversity in union modes and nuclear events of spathidiids; E. ascendens even has preconjugation division. Confirming previous data, the present study shows convincingly that most of the spathidiid nuclear variability is caused by reconstruction processes occurring in post-dividers, exconjugants and, possibly, exautogamonts. When these specimens are removed from the populations, spathidiid species are as stable (or variable) as other ciliate species.