In Ulva rigida the study of relationships between total nitrogen content (N), total carbon content (C), and C/N ratio, show that two different patterns are determined by the relations between N, C and C/N ratio. The first pattern is characterized by a very rapid decrease in the C/N ratio when N increases, an oscillatory relation between C and N and also between C/N and C. This pattern was characteristic of experiments where algae suffered nitrogen starvation for several days. The second pattern is characterized by a considerably smaller decrease of CM ratio when N increases, a linear relationship between C and N, as well as between C/N ratio and C, but with a slope of practically zero. This pattern was characteristic of experiments with algae that did not suffer any nitrogen starvation period. The observed behavior is explained in terms of the difference between the rate of increase in nitrogen content and that of increase in carbon content in both physiological situations. When algae suffered a period of nitrogen starvation, the change in the rate of nitrogen assimilation was higher than that of carbon assimilation as was shown by the quick decrease of C/N ratio. A strong imbalance between the N and C increase rates, as caused by NO₃^- supply to nitrogenstarved algae, produces a transition by oscillations to a new steady state situation characterized by a smaller C/N ratio. In the new steady state, between the increase in N and the increase in C, a linear relationship appears with a defined constant stoichiometry between carbon metabolism and inorganic nitrogen metabolism, in a wide range of carbon and nitrogen content. In this situation algal growth is not limited by nitrogen supply.