Abstract Recurrent blooms of toxic cyanobacteria dominate surface waters of the western Gulf of Finland in late summer, overlapping peak densities of two copepods, Acartia bifilosa (Giesbrecht) and Eurvtemora affinis (Poppe). These cyanobacteria possess morphological and toxic properties that might affect planktic herbivory and impact secondary production. In summer 1994 clearance rates of the two copepods on the cyanobacteria Nodularia spumigena Mertens and Aphanizomenon .flosaquae Raifs ex Bornet et Flahault were measured over a range of phytoplankton densities. The effects of the two cyanobacteria on copepod egg production were also investigated. Clearance rates of the two copepods feeding on cyanobacteria were substantially lower than the rates of copepods fed similar levels of cyanobacteria-free food, the flagellate Brachiomonas submarina Bohlin, or < 63 μm ambient phytoplankton assemblages. In addition, cyanobacteria-rich assemblages reduced egg production in both copepods, with rates declining from...