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Current estimates of viral abundance in natural waters rely on direct counts of virus-like particles (VLPs), using either transmission or epifluorescence microscopy. Direct counts of VLPs, while useful in studies of viral ecology, do not indicate whether the observed VLPs are capable of infection and/or replication. Rapid decay in bacteriophage viability under environmental conditions has been observed. However, it has not been firmly established whether there is a corresponding degradation of the virus particles. To address this question, viable and direct counts were carried out employing two Chesapeake Bay bacteriophages in experimental microcosms incubated for 56 h at two depths in the York River estuary. Viruses incubated in situ in microcosms at the surface yielded decay rates in full sunlight of 0.11 and 0.06 h-1 for CB 38 phi and CB 7 phi, respectively. The number of infective particles in microcosms in the …