The growth and CO2 concentrating mechanism of a filamentous cyanobacterium Anabaena cylindrica IAM-M1 in a bubble column have been studied by following the time courses of biomass concentration, pH, CO2 accumulation rate, and transmittance of light. The biomass production increased with the increasing superficial gas velocity from 9.0 m.h(-1) to 12 m.h(-1) or with increasing mole fraction of CO2 at the inlet from 0.03 to 0.06, while it decreased with the increasing superficial gas velocity from 12 m.h(-1) to 18.0 m.h(-1) or with the increasing fraction of CO2 from 0.06 to 0.12. As biomass concentration increased the photosynthetic photon flux density of the transmitted light decreased. The CO2 accumulation rate, which almost equaled in magnitude the CO2 mass transfer rate (CTR), went up steeply in the early phase of cell growth. The photosynthetic CO2 uptake rate (CUR) reached the maximum at the late logarithmic growth phase. The maximum value of the ratio CTR/CUR was 804.