The article reviews current knowledge on the tripartite symbiosis: Paramecium bursaria–Chlorella sp.–virus PBCV. Special attention is paid to the studies initiated by Boris Gromov, and continued later, without his participation, at the Microbiology Department of St. Petersburg/Leningrad State University. Virus PBCV (Paramecium bursaria Chlorella virus), the type species of the genus Chlorovirus (Phycodnaviridae), belongs to the NCLDV group (nucleo-cytoplasmic large DNA-containing viruses). It has 100–400 nm size capside and 0.3–1.2 Mbp dsDNA, and is highly specific, namely: only endosymbiotic algae (zoochlorellae) are infected. Moreover, the algae that stay within perialgal vacuoles are safe from infection while outside the damaged or dead host they are infected by absorbed virus particles. In a sense, PBCV takes position at the host’s border, and “watches” on appearing algae (hence, figurative comparison with an ambush predator). In Russia, the symbiosis has been studied for three decades by the group headed by Gromov’s collaborator Professor Konstantin V. Kvitko, and his partners. During this period of time (late 1980s – late 2010s), various aspects of tripartite symbiosis have been analyzed, such as: i) geographic distribution of the “American” (“south”) and “Eurasian” (“north”) ecotypes; ii) difference in serological characters and protein composition between different zoochlorellae ecotypes; iii) phylogenetic distinction of zoochlorellae deduced from rRNA genes similarity, and intron structure; iv) role of host exometabolites in the symbiosis; v) restoration of symbiotic system from algal strains marked with resistance to antibiotics; vi) ecological role of PBCV, as well as its influence on host evolution; vii) practical significance of PBCV.