A population ofEuplotescollected from the mouths of the Rhône was assigned to the morphospeciesE. charon.The 27 strains established in the laboratory were found capable of producing protective cysts in response to environmental salinities exceeding 502. and, with the exception of two, all of them formed mating pairs. Mating interactions appeared to be controlled by a system of multiple mating types, probably inherited via alleles condominant at the genetic locusmat.Cells in the G1cell‐cycle stage, which occupies 62% of the generation time, as well as cells in the S stage could mate. Mating was followed by an unusually long period of immaturity of 120–180 fissions. In the conjugants, the micronucleus performed four divisions, one preliminary and three pregamic; the macronucleus persisted unchanged. Cytological observations suggested that the two gamete nuclei derive from different products of the preliminary micronuclear mitosis. The synkaryon regularly performed three metagamic divisions and, in the exconjugants, the old macronucleus underwent complete resorption following an unusual phenomenon of positional inversion. The first metagamic division was determinative for the origin of the new macronucleus and micronucleus.