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We examined changes in morphological and genomic diversities of viruses by means of transmission electronic microscopy and pulsed field gel electrophoresis (PFGE) over a nine-month period (April-December 2005) at four different depths in the oligomesotrophic Lac Pavin.We found that the majority of viruses in this lake belonged to the family of Siphoviridae or were untailed,with capsid sizes ranging from 30 to 60 nm,and exhibited genome sizes ranging from 15 to 45 kb.On average,12 different genotypes dominated each of the PFGE fingerprints.The highest genomic viral richness was recorded in summer (mean =14 bands per PFGE fingerprint) and in the epilimnion (mean =13 bands per PFGE fingerprint).Among the physico-chemical and biological variables considered,the availability of the hosts appeared to be the main factor regulating the variations in the viral diversity.