Conjugation in Stentor coeruleua was investigated using two experimental methods which regularly yielded large numbers of mating pairs. One method involves a particular culture technique, the other requires a mixture of cells from different stocks. Mating pairs appeared in the form of bursts of conjugation, either induced by mixing certain stocks or occurring spontaneously in some stock cultures. Spontaneous bursts, in the majority of cases, occurred during a definite interval in the development of a culture. Morphologically distinct pre-conjugator cells appear immediately before as well as during the initial stages of a burst of conjugation. Mating pairs were formed by the union of two pre-conjugators. Mixing eight stocks in all possible combinations of twos and observing their subsequent response revealed they were separable into two complementary mating types. The majority of mating pairs formed in mixtures of stocks consisted of individuals of different mating types. Evidence is presented which is compatible with the hypothesis that cell to cell contacts between individuals of differing mating type are necessary for the initiation of a mating reaction. / Science, Faculty of / Zoology, Department of / Graduate