Streptomyces ambofaciens is prone to genetic instability involving genomic rearrangements at the extremitiesof the chromosomal DNA. An amplified DNA sequence (ADS205), including an open reading frame (orfPS), isresponsible for the reversible loss of spiramycin production in the mutant strain NSA205 (ADS2051 Spi2). Theproduct of orfPS is homologous to polyketide synthase systems (PKSs) involved in the biosynthesis of erythromycinand rapamycin and is overexpressed in strain NSA205 compared with the parental strain RP181110.As PKSs and fatty acid synthase systems have the same precursors, we tested the possibility that overexpressionof orfPS also affects lipid metabolism in strain NSA205. This report focuses on comparative analysis oflipids in strain RP181110, the mutant strain NSA205, and a derivative, NSA228 (ADS2052 Spi1). NSA205showed a dramatically depressed lipid content consisting predominantly of phospholipids and triacylglycerols.This lipid content was globally restored in strain NSA228, which had lost ADS205. Furthermore, strainsRP181110 and NSA205 presented similar phospholipid and triacylglycerol compositions. No abnormal fattyacids were detected in NSA205.