Recent evidence suggests that viruses play an influential role within the marine microbial food web. To understand this role, it is important to determine rates and mechanisms of virus removal and degradation. We used plaque assays to examine the decay of infectivity in lab-grown viruses seeded into natural seawater. The rates of loss of infectivity of native viruses from Santa Monica Bay and of nonnative viruses from the North Sea in the coastal seawater of Santa Monica Bay were determined. Viruses were seeded into fresh seawater that had been pretreated in various ways: filtration with a 0.2-(mu)m-pore-size filter to remove organisms, heat to denature enzymes, and dissolved organic matter enrichment to reconstitute enzyme activity. Seawater samples were then incubated in full sunlight, in the dark, or under glass to allow partitioning of causative agents of virus decay. Solar radiation always resulted in increased rates of loss of virus infectivity. Virus isolates which are native to Santa Monica Bay consistently degraded more slowly in full sunlight in untreated seawater (decay ranged from 4.1 to 7.2% h(sup-1)) than nonnative marine bacteriophages which were isolated from the North Sea (decay ranged from 6.6 to 11.1% h(sup-1)). All phages demonstrated susceptibility to degradation by heat-labile substances, as heat treatment reduced the decay rates to about 0.5 to 2.0% h(sup-1) in the dark. Filtration reduced decay rates by various amounts, averaging 20%. Heat-labile, high-molecular-weight dissolved material (>30 kDa, probably enzymes) appeared responsible for about 1/5 of the maximal decay. Solar radiation was responsible for about 1/3 to 2/3 of the maximal decay of nonnative viruses and about 1/4 to 1/3 of that of the native viruses, suggesting evolutionary adaptation to local light levels. Our results suggest that sunlight is an important contributing factor to virus decay but also point to the significance of particles and dissolved substances in seawater.