Carbon and nitrogen contents of two intertidal fucoid species, Fucus serratus and Himanthalia elongata, were investigated with respect to variations in seasonal resource availability, growth and reproductive requirements. The linear growth rate of F. serratus peaked in spring at 2·3 cm 28 d−1, compared with <0·1 cm 28 d−1 in the winter. In H. elongata, the button diameter increased slowly throughout the year (<0·22 cm 28 d−1) ; in contrast, the receptacle had an elongation rate of up to 7·8 cm 28 d−1 in the spring months. There was no difference in the nitrogen content (% dry weight, dwt) of the vegetative tissue of both non-reproductive and fertile thalli and receptacle tissue of F. serratus, but the nitrogen content of all three tissue types varied seasonally. Reproductive development was initiated in May when nitrogen content was at its peak (3 % dwt). Tissue nitrogen content decreased rapidly through reproductive development to a minimum of less than 1·5% dwt in August; this decrease also occurred in non-reproductive thalli. Tissue nitrogen varied between 0·5 and 1·75% dwt in the vegetative buttons on both non-reproductive and fertile H. elongata, but not in a distinct seasonal manner. Receptacle development in H. elongata was initiated in October/November. The nitrogen content of the receptacle tissue increased rapidly in the first two months of reproductive development (up to 2·5% dwt) then progressively decreased throughout the remaining period of reproductive development. There was no evidence of carbon storage in the vegetative tissues of either F. serratus or H. elongata.