Abstract: Based on a cyanophage PaV-LD (Planktohtrix agardhii Virus isolated from Lake Donghu) infecting bloom-forming filamentous cyanobacterium Planktothrix agardhii from a typical shallow freshwater lake—Lake Donghu, the impact of host cyanobacterial culture conditions on the cyanophage efficiency and lytic activity of this cyanophage was analyzed in this study. PaV-LD was inoculated into cyanobacterial cultures of host under different states, such as growth phase, renewal rate of semicontinuous cultures and illumination condition, and the burst size of progeny cyanophage and host cell lysis efficiency of host cyanobacteria was determined by the most-probable number (MPN) method and transmission electron microscopy (TEM) observation, respectively. The results showed that the average burst size of infectious progeny PaV-LD released from each infected host cell at exponential phase was 350 infectious units (IU)/cell, which was significantly higher than that of each infected host cell at stationary phase (110 IU/cell); the initial cell densities of host cultures at exponential phase and stationary phase was 1.38×106 cells/mL and 1.43×107 cells/mL, and respectively decreased to 1.75×103 cells/mL and 3.0×103 cells/mL at 5 days after infection, indicating that no significant difference was found in the sensitivity of host cell at different growth stage to PaV-LD infection. Semicontinuous culture experiment was performed, either 0, 35%, 50% and 65% of the fresh BG11 medium was replaced and 5 days after infection, the burst size of PaV-LD were estimated to be ca. 50 IU/cell, 70 IU/cell, 220 IU/cell and 310 IU/cell. Thus, the burst size of PaV-LD increased with the addition of medium renewal rate in a certain range. Infected host cells were lysed under light 3 or 4 days later. Numerous progeny PaV-LD was observed with TEM. Additionally, the titer aggrandized rapidly from 7.0×103 IU/mL to 8.56×107 IU/mL during this period. On the contrary, the infected host cells were not cracked under darkness. Simultaneously, progeny PaV-LD could not be detected. Further, TEM observation showed that thylakoid membranes of infected host cells under light were disappeared, and progeny PaV-LD were mainly distributed in the thylakoid location of the original. The results showed that cyanobacteria cell culture conditions might play a decisive role in obtaining pure cultures of the cyanophage