Pelagic unicellular marine green algae in the class Mamiellophyceae are abundant and widespread in the oceans, and numerous complete genomes of these species are now available. They are often infected by prasinoviruses, probably the most numerous kind of large DNA viruses of eukaryotes in the sea. In the laboratory, compatible viruses lyse algal cultures, but spontaneously resistant cell lines arise frequently and are either immune (no virus present), or chronically infected (producing viruses at a low level). We have produced 38 independent lines of Ostreococcus tauri resistant to OtV5, and we are currently monitoring their phenotypes and genotypes in culture. The majority of them initially remain chronically infected for about a year, but over 2 years more and more become immune, resistant and free of viruses. Transcriptomic analyses revealed a sweeping increase in gene expression spanning a region of more than 170 kb on one chromosome, and karyotypic analysis by PFGE showed that physical reorganization of this chromosome had occurred in nearly 36/38 of the virus-resistant lines.