Abstract.Giemsa stained blood smears prepared from blennies caught on the shores of southern England and from cod taken from the Celtic and North seas revealed fish with clinical ‘piscine erythrocytic necrosis’ (PEN). The findings recorded here extend the distribution data on PEN infections and report that the infection in the blenny and cod may possibly have a similar developmental pattern. Data are presented on the prevalence of the infection in the fish population samples and on the proportion of infected cells in the smears exami