We compared seasonal changes in tissue C, tissue N, C:N ratio, and total phenolic acids for five species of submersed plants from Fall River, California. Mean tissue C differed among the five species and was between 35 and 40%, except forRanunculus aquatilis, which was 26%. Mean tissue N differed among species;Ranunculus aquatilishad the lowest mean value (2.71%) andZannichellia palustrishad the highest (3.74%). Mean C:N ratio was between 9.7 and 12.7. Among the five species examined, mean phenolic acid content was greatest forMyriophyllum sibiricumand it was nearly seven times that for the species with the lowest phenolic acid content,Elodea canadensis. Regression analysis indicated positive relationships between phenolic acid content and tissue C for two species,Callitriche hermaphroditicaandRanunculus aquatilis. Regression equations relating phenolic acid content to tissue N were not statistically significant. ForElodea canadensis, the equation describing phenolic acid content as a function of the C:N ratio had a negative slope, while the equation forCallitriche hemaphroditica, had a significant positive slope. Knowledge of the relationships between plant nutrient content and phenolic acids may lead to better understanding of aquatic plant/herbivore/microbe interactions and might result in improvements in the timing of applications of biological control agents in aquatic plant management.