This issue contains papers presented at the Fifth International Workshop on the Measurement of Microbial Activities in the Carbon Cycle in Aquatic Environments. The Workshop was held in Helsingor, Denmark, from 18 to 23 August, 1991, and was organized by The Water Quality Institute and the Marine Biological Laboratory at University of Copenhagen. The preceeding workshops in this series were held in Plön (Germany) 1977, at Malente (Germany) 1983, in Utrecht (The Netherlands) 1986, and in Ceske Budëjovice (Czechoslovakia) 1988. Proceedings from these workshops were published in Arch. Hydrobiol. Beih. Ergebn. Limnol. 12 (1979), 19 (1984), 31 (1988) and 34 (1990). The organizers decided to focus this fifth workshop on the microbial ecology of pelagic environments. Despite this narrowing of the scope, the workshop still attracted a higher number of participants than previously, and the 140 participants were in for a tight programme. The series of workshops have reflected a general trend in the research development. While basic methodological problems dominated the early workshops, this workshops revealed a further development into a more mature stage where data collected from a variety of pelagic environments were integrated in an ecological context.