The fresh water eustigmatophyte Monodus subterraneus is one of the most promising of the microalgal sources of eicosapentaenoic acid (20:503, EPA). EPA is generally the major fatty acid. Depending on cultivation conditions the proportion of EPA in this species ranges from 20 to 37 per cent of total fatty acids (Table 2.1). Other major fatty acids are, in decreasing order of occurrence, 16:lw7, 16:O and 18:1w9.