Chemosensory Responses Toward Oxygen in Euglena gracilis*

SYNOPSIS Euglena gracilis strain Z, at a concentration of 106 cells/ml and in containers of ∽ 0.1‐mm thickness, spontaneously forms dynamic ring patterns in the dark. These patterns are modified differentially by illumination with red and with blue light. The red light effect is abolished by treatment with an inhibitor of photosynthesis. Pattern formation is apparently the result of chemophobic responses to oxygen dissolved in the medium. Euglena can respond to both negative and positive concentration gradients, depending upon the absolute magnitude of oxygen concentration. The photo‐ and chemosensory transduction systems of Euglena interact at a stage which precedes the overt expression of motor responses.