The accumulation of polyunsaturated fatty acids by algae Spirulina platensis and Chlorella minutissima was studied. Response of these organisms to the substituted pyridazinone, SAN 9785, an inhibitor of the long chain fatty acid desaturase, indicated that fatty acid synthesis and their desaturation were regulated differently in these organisms. While the pool of palmitic acid, the precursor for the unsaturated C₁₈ fatty acids, was stringently maintained in the green alga C. minutissima, in the cyanobacterium S. platensis the level of palmitic acid was liberally maintained in spite of the enhanced accumulation of unsaturated C₁₈ fatty acids.