In both stocks R and W of P. aurelia, endomixis can be induced by placing at 31° C. small mass cultures containing the surplus animals from isolation lines. In stock R, induction cultures set up soon after endomixis yielded conjugants, so that the induction of endomixis could not be studied quantitatively in these. Later cultures, however, showed increasing percentages of induction until 100 per cent was obtained after one day at 31° C. Earlier cultures which gave but low percentages of endomixis with this treatment could be induced to give larger percentages by subculturing or adding some fresh culture medium to the original induction culture. In a few exceptional cases, abnormal reorganization of the nuclear apparatus was soon followed by a normal endomictic reorganization in isolation lines, and by the induction of endomixis instead of conjugation in the induction cultures set up immediately after the abnormal reorganization. In stock W, the absence of conjugation made it possible to examine the percentages of endomixis induced at all periods of the interendomictic interval. During the first few days after an endomixis, it required prolonged treatment to induce even a low percentage of endomixis, but as the time since the last endomixis increased, higher and higher proportions of endomixis were induced with shorter and shorter treatments, until 100 per cent endomixis could be induced in two days. Irregularities in this progression are probably due to environmental differences in treatment from day to day. Rise of temperature above 31° C. was shown to reduce the percentage of endomixis induced. Other conditions also probably play a similar rôle. Stocks R and W differed in two respects in regard to their reaction to the conditions favoring the occurrence of endomixis. (1) In stock W, endomixis could regularly be induced 7 days after a preceding endomixis, and in some cases, as early as the third day; but in stock R, the typical response at this period was conjugation, not endomixis. (2) Late in the series of induction cultures, 100 per cent endomixis could be induced with a treatment of one day in stock R, but treatment of two days was required in stock W.