Two methods of heritability estimation for threshold character were compared to clarify merits and demerits. The data on mortality in the fry stage of masu salmon originating from six mature females and ten mature males caught in the Shokanbetsu River in Hokkaido were used. The heritabilities were estimated by two methods, i.e. the coding method where 0 or 1 values are assigned to dead and alive fry, and the arcsine transformation method where percentage data of dead fry obtained from lots of two series of experiments for six half sibs as to distinct females and ten half sibs as to distinct males are used. From the results two points can be emphasized. One is that heritabilities for initial mortality rate are minute and almost zero, suggesting the action of strong environmental influences. The other is that the coding method can be used to estimate the heritability instead of the arcsine transformation method for threshold character.