Beschreibung:
<jats:title>SYNOPSIS</jats:title><jats:p>Responses of <jats:italic>Paramecium bursaria</jats:italic> to light intensity changes were investigated. The resting paramecia show a direction changing response (photophobic response) to a sudden decrease of light intensity, whereas no response was shown to an increase in intensity. The critical intensity decrease dI<jats:sub>c</jats:sub> necessary to show the response was measured at various values of initial light intensity, and the ratio dI<jats:sub>c</jats:sub>/I was found to be equal to ∼0.15. The swimming paramecia show different behavior depending on their swimming direction in the spatial gradient of light intensity. Paramecia show direction change more frequently when they are swimming down the gradient than in the opposite direction. This difference in the rate of direction changing is 13–17%. These results may offer an explanation for the mechanism of photoaccumulation.</jats:p>