1. The quantitative relationship between chemoaccumulation and locomotor activity in specimens of Paramecium caudatum was examined by employing a novel chemoaccumulation assay system. 2. Distribution of the specimens in two adjoining solutions with different chemical compositions was dependent on both the rate of arrival of the specimens at the border between two solutions (the arrival rate) and the ratio of the number of specimens crossing the border to that arriving at the border in unit time (the pass ratio). 3. The arrival rate was higher when the forward swimming velocity was higher and when the frequency of spontaneous avoiding responses was lower. The pass ratio was higher when the degree of the avoiding response at the border was lower. 4. The translocation rate was defined as the number of specimens crossing from one solution to another in unit time, corresponding to the product of the arrival rate and the pass ratio. 5. In the steady-state condition, the distribution ratio, which is defined as the ratio of the number of specimens in the test solution to that in the reference solution, was proportional to the ratio of the translocation rate towards the test solution from the reference solution to that towards the reference solution from the test solution. 6. When specimens showed a higher arrival rate in one solution than the other the pass ratio was lower, so that the arrival rate and the pass ratio acted antagonistically in establishing chemoaccumulation. 7. The involvement of membrane-potential-mediated changes in the motile activity of specimens in establishing chemoaccumulation is discussed.

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