Multiwell plates are convenient for isolating cyanophages from environmental samples using the liquid bioassay approach (Table 1). This protocol describes a typical procedure (96-well assay) used to detect and isolate cyanophages from marine samples that lyses Synechococcus sp. strain DC2 (also known as CCMP1334 or WH7803). Using this method, greater than 105 lytic phages per milliliter of seawater have been detected in the Gulf of Mexico (Suttle and Chan 1994) that lyses this permissive target host.