Paramecium bursaria (green paramecium) possesses endosymbiotically growing chlorella-like green algae. An aposymbiotic cell line of P bursaria (MBw-1) was prepared from the green MB-1 strain with the herbicide paraquat. The SA-2 clone of symbiotic algae was employed to reinfect MBw-1 cells and thus a regreened cell line (MBr-1) was obtained. The regeened paramecia were used to study the impact of the host's growth status on the life cycle of the symbiotic algae. Firstly, the relationship between the tinting of algal propagation and the host cell division was investigated by counting the algal cells in single host cells during and after the host cell division and also in the stationary phase. Secondly, the changes in the endogenous chlorophyll level, DNA content, and cell size in the symbiotic algae were monitored by flow cytometry and fluorescence microscopy. The number of algae was shown to be doubled prior to or during the host cell division and the algal population in the two daughter cells is maintained at constant level until the host cell cycle reenters the cytokinesis, suggesting that algal propagation and cell cycle are dependent on the host's cell cycle. During the host's stationary growth, unicellular algal vegetatives with low chlorophyll content were dominant. In contrast, complexes of algal cells called sporangia (containing 1-4 autospores) were present in the logarithmically growing hosts, indicating that algal cell division leading to the formation of sporangia with multiple autospores is active in the dividing paramecia.