Diet variations in substrate availability (largely due to variations in phytoplankton production) can lead to pronounced diels patterns in prokaryotic activity in the euphotic zone. We examined short-term changes in viral infection of bacterioplankton and its relation to bacterial activity in 3 distinct masses of North Sea surface waters marked by drifting buoys in June 2001 and April 2002. The water masses were sampled every 4 to 6 h for a period of 20 to 36 h. The frequency of infected cells (FIC) estimated by a virus dilution approach varied from 5 to 64% at the western site, 17 to 55% at the southern site, and 10 to 22% at the northern site and was generally higher during the night than at daytime. Furthermore, FIC was negatively related to bacterial activity at all sites. Bacterial activity, measured via [C-14]-leucine incorporation, was ca. 1.5- to 5-fold higher during the day than at night. Our results indicate that viral lysis of bacteria occurs around noon to afternoon, and infection mainly during the night. Moreover, lysis and viral production seem to take place during high bacterial activity; this could be a strategy to increase the number of newly produced viruses.