Relations among carbon and nitrogen contents, photosynthetic capacity and surface-to-volume ratio were investigated using the discs cut from sporophytes of Laminaria japonica Areschoug. The relation between the carbon and nitrogen contents was divided into three phases on the basis of two distinctive nitrogen content levels, namely the subsistent and critical values. The first phase was characterized by a positive correlation between carbon and nitrogen amounts when nitrogen level was above the critical value (2.1% of dry weight). In the second phase, when nitrogen content decreased from 2.1% to 1.3% of the subsistent content, the correlation between carbon and nitrogen levels was negative. During the last phase, when the nitrogen content decreased below the subsistent value, carbon accumulation stopped. On the contrary, the photosynthetic activity of sporophytes under optimal conditions was severely suppressed when the nitrogen content fell below the subsistent value. Nitrogen levels above the subsistent value resulted in the linear increase in photosynthetic activity. In addition, the surface-tovolume ratio was correlated to the photosynthetic activity. These results indicate that photosynthetic capacity, carbon content and tissue size are linked to the nitrogen status, thussuggesting that the critical and subsistent nitrogen contents are helpful indicators for elucidating the productivity of sporo-phytes.