The herbicide Norflurazone was shown to be an inhibitor of fatty acid of the Delta6 desaturation system. Treating cultures of the microalgae Spirulina platensis or Monodus subterraneus with this herbicide brought about a reduction in the level of gamma-linolenic acid and in an increase in the level of linoleic acid. In Monodus, the increase in linoleic acid made it more available for omega3 desaturation, resulting in an increase in the proportion of eicosapentaenoic acid. The proportion of arachidonic acid did not decrease albeit the drastic decrease in its precursor gamma-linolenic acid.