More than 40 years ago, Safferman and Morris (1963) used the plaque assay method to isolate the first cyanophage that infects a freshwater filamentous cyanobacterium, Plectonema boryanum. Since then, this approach has been used successfully to detect and isolate a number of different phages infecting marine Synechoccocus and Prochlorococcus (e.g., Suttle and Chan 1993; Waterbury and Valois 1993; Wilson et al. 1993; Sullivan et al. 2003).The advantages of this method are that results are easily interpreted as plaques formed on pigmented lawns can be easily identified. Since a plaque is the result of a single infection event, the virus can be easily purified and cloned.\u00a0The first part of the procedures described as follows: (1) preparation of base plates and (2) preparation of top agar\/agarose are applicable also for bacteriophage plaque assays.