A previous report indicated that prototype chlorovirus PBCV-1 replicated in two Chlorella variabilis algal strains, NC64A and Syngen 2-3, that are ex-endosymbionts isolated from the protozoan Paramecium bursaria. Surprisingly, plaque-forming viruses on Syngen 2-3 lawns were often higher than on NC64A lawns from indigenous water samples. These differences led to the discovery of viruses that exclusively replicate in Syngen 2-3 cells, named Only Syngen (OSy) viruses. OSy-NE5, the prototype virus for the proposed new species, had a linear dsDNA genome of 327kb with 44-nucleotide-long, incompletely base-paired, covalently closed hairpin ends. Each hairpin structure was followed by an identical 2612 base-paired inverted sequence after which the DNA sequence diverged. OSy-NE5 encoded 357 predicted CDSs and 13 tRNAs. Interestingly, OSy-NE5 attached to and initiated infection in NC64A cells but infectious progeny viruses were not produced; thus OSy-NE5 replication in NC64A is blocked at some later stage of replication. Copyright © 2016 Elsevier Inc. All rights reserved.