Trichomes of an Oscillatoria sp. suspended in soft agar in Petri dishes showed active movement away from areas where gaseous exchange had been prevented by placing glass coverslips on the surface. This clearance response occurred only in the light and it was related to gradients of inorganic carbon that formed in the agar layer. In plates incubated in the dark trichomes accumulated in the central areas below glass coverslips but the substance eliciting this response could not be identified. Gradients of diffusible substances were established within lawns of cyanobacteria suspended in soft agar and it was demonstrated that trichomes of Oscillatoria sp. moved towards CO2. HCO3 and O2 in light-dependent chemotactic reactions. No chemotaxis occurred in response to these substances in the dark. The chemotactic responses were detected after 2 h but became increasingly distinct up to 8 h. The chemotactically active concentration of CO2 was greater than the atmospheric concentration since trichomes moved from air towards a source of CO2. Using diffusivity coefficients it was calculated that trichomes of Oscillatoria sp. accumulated at a CO2 partial pressure of 0.02 bar (equivalent to 0.83 mM). With O2 as the chemoattractant the value was 0.35 bar (14.56 mM). These results are discussed with reference to the roles of inorganic carbon and O2 in cyanobacterial metabolism and it is concluded that chemotactic behaviour may be important in movements within the photic zone of sediment environments.