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Viruses infective to the marine coccolithophore Emiliania huxleyi were isolated from waters of the Gulf of Maine during May and June of 2004, a period when ambient E. huxleyi concentrations were minimal. Three DNA-containing, ether-insensitive virus isolates possessed icosahedral symmetry, and were 130 to 160 nm in diameter. All isolates caused complete lysis of host cultures within 4 d, produced large plaques on host lawns in agarose and were stable at -72°C for at least 18 mo. Originally propagated on the 88E strain of E. huxleyi, none of the viral isolates were able to infect related host strains, including several that were native to the Gulf of Maine. They were also noninfectious for strains of Synechococcus and Micromonas pusilla. One-step growth studies showed a high level of virus propagation during the first 24 h, a period when corresponding host populations were quite stable. This finding led us to speculate that virus progeny might be leaking from intact hosts during this period, with eventual host lysis occurring later. The viruses appeared to be highly infectious, with multiplicities of infection (MOI) as low as 10 -5 clearing host cultures within 96 h. Similar efficiencies were seen when host concentrations were as low as 10 2 cells ml -1