A pedigreed race of P. aurelia, whose culture began on May 1, 1907, was carried on by almost daily isolations until May 1, 1915, the 5071st generation having been reached. The race was continued with less rigid examinations until April, 1926. Estimates indicate that there were 600 generations per year, or a total of 11,700 generations during 19 years. The continued vitality of this culture is indicated by the fact that paramecia are all the time being taken from the culture for experimental purposes. No conjugation has been observed, but rhythms in the fission rate occur, correlated with a reorganization of the nucleus, termed endomixis. The data to date indicate that fertilization is not requisite in the life history of the organism, if the environmental conditions are wholly favorable, although fertilization may have a stimulating effect.