Lysogeny was established in Plectonema boryanum, a filamentous blue-green alga, by LPP-1D and LPP-2 phycoviruses after treatment of the alga with antibiotics, heavy metal, and nutrient limitation. Mitomycin C induction and viral neutralization confirm lysogeny. Infection of antibiotic treated cells by LPP-1 virus failed to establish lysogeny, indicating that LPP-1 is a virulent phycovirus while LPP-1D and LPP-2 were found to be temperate viruses. Continuous culture techniques were also used to study long-term population interactions of Plectonema and the LPP-viruses. After LPP-1 infection of sensitive algae, three oscillations occur in cell density with concomitant increases in virus before final stabilization of both algal and phycoviral populations. After LPP-1D and LPP-2 infection, only one fall in cell density occurs with a burst of virus particles. Resistant algae always repopulated; lysogeny was never established in chemostat experiments.