Juvenile coho salmon (Oncorhynchus kisutch) fed a folic acid-deficient diet and sampled at 6, 9, 12, and 14 weeks developed macrocytic anemia. The anemia, first observed at 6 weeks, was characterized by a significant reduction in red blood cell count as well as macrocytosis and poikilocytosis of erythrocytes. The abnormally shaped erythrocytes observed in peripheral blood smears may be an important aid in the identification of folic acid deficiency in coho salmon. Gross manifestations of the deficiency were extremely pale gills; exophthalmia, often accompanied by ascites fluid; dark coloration; and reduction in growth. Fish recovered after 8 weeks on a diet adequate in folic acid and exhibited a normal blood picture.