Electron-microscopic examination of water samples from the hypersaline Dead Sea showed the presence of high numbers of virus-like particles. Between 0.9 and 7.3 x 10(7) virus-like particles ml(-1) were enumerated in October 1994 in the upper 20 m of the water column during the decline of a bloom of halophilic Archaea. Virus-like particles outnumbered bacteria by a factor of 0.9-9.5 (average 4.4). A variety of viral morphologies were detected, the most often encountered being spindle-shaped, followed by polyhedral and tailed phages. In addition, other types of particles were frequently found, such as unidentified algal scales, and virus-sized star-shaped particles. Water samples collected during 1995 contained low numbers of both bacteria and virus-like particles (1.9-2.6 x 10(6) and 0.8-4.6 x 10(7) ml(-1) in April 1995), with viral numbers sharply declining afterwards (less than 10(4) ml(-1) in November 1995-January 1996). It is suggested that viruses may play a major role in the decline of halophilic archaeal communities in the Dead Sea. an environment in which protozoa and other predators are absent.