ABSTRACT: The major groups of biochemical compounds (proteins, lipids, carbohydrates, chitin, nucleic acids and nucleotides) have a distinct functional role and, therefore, incorporation of biochemical interpretations into stoichiometric analysis would improve our current understanding of species life histories and their interactions. A review of the stoichiometric composition of the main biochemical compounds of freshwater and marine zooplankton revealed high constancy. No significant differences were found between marine and freshwater species or between any of the taxonomic groups considered (copepods, cladocerans, euphausiids and mysiids) for most of the main biochemical compounds (triacylglycerol, wax esters, phospholipids and proteins). Therefore, it can be concluded that inter- and intraspecific stoichiometric variability is a result of differences in the proportions of the main biochemical compounds. Comparison of the biochemical composition of 182 freshwater and marine species revealed that freshwater species had lower ATP and free amino acid content and higher RNA content than marine species. The elemental composition of a single copepod species, Calanus finmarchicus, and the averages of each taxonomic group were estimated from the composition of major biochemical compounds and compared with the measured elemental content. Almost all of the measured carbon (C), nitrogen (N) and phosphorus (P) were accounted for. After proteins, nucleic acids were the second most abundant compound in the N pool of freshwater species, while free amino acids were the second most prevalent compound in marine species. Phospholipids were the most abundant compound in the P pool of marine species, while in freshwater species nucleic acids were the most abundant compound.