Our paper describes case of lerneosis in rainbow trout (Oncorhynchus mykiss). Lerneosis is disease of fish caused by Lernaea cyprinacea. This species is a cosmopolitan ectoparasite, worldwide distributed. Invasion by L. cyprinacea were rare described in Poland. This parasite is highly adapted to a parasitic life. The life cycle of L. cyprinacea is composed of nine stages. After mating males (mature stage) subsequently die and mated females attach to the body surfaces of host fish and then passing trough the skin to the muscles. Infections of this parasite can cause localized inflammation and ulceration at the area of attachment and are entrance of secondary bacterial and fungal infections. Massive invasion in fish can cause serious economic problems.

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