The lavers (Porphyra tenera Kellm.) are usually attacked by the wasting disease during the culture period in sea. It is rational to discuss the causes from two different categories: One is non-parasitic, and the other is parasitic. Among the former cases, “Shiro-Gu_??_ari-Byo” (white wasting disease) seems to be most harmful. The disease is suggested to be caused by the unfavourable conditions for the alga, such as the lack of uncovered and drying periods in the sea. In the parasitic diseases, “Aka-Gusari-Byo” (red wasting disease) gives the greatest damages. The pathogenetic fungus is though_??_ to be a new species of a new genus allied to the pythium, but whose name is not yet determined. The life-history and the eco-physiological properties of the fungus were studied in details.