- Abstract The effects of the herbicides SAN 9785 and norflurazon on growth and fatty acid composition of the cyanobacterium Spirulina platensis , the marine microalga Porphyridium cruentum and the freshwater alga Monodus subterraneus were studied. Norflurazon was more effective than SAN 9785 in causing growth inhibition of S. platensis cultures; changes in fatty acid composition confirmed that this compound is a potent inhibitor of Δ6-desaturation. SAN 9785 inhibited the growth of S. platensis concomitantly causing a reduction in the amount of the fatty acids 18:3ω6 and 18:2ω6 and an increase in 18:1 and 18:0, thus indicating an inhibitory effect of this herbicide on the Δ6-desaturation system. Similar growth inhibition was observed in P. cruentum . In this alga, the herbicide also inhibited the formation of 20:5ω3, apparently by blocking the desaturation of 18:2 to 18:3ω3 in one pathway of eicosapentaenoic acid biosynthesis. However, in M. subterraneus SAN 9785 reduced growth, but caused an increase in the proportion of 20:5ω3 indicating a possilbe alternative pathway for the formation of 20:5ω3 in this alga. SAN 9785 was therefore considered to be useful for the selection of cell lines which overproduce the valuable fatty acids 18:3ω6 and 20:5ω3 in S. platensis and P. cruentum , respectively.