Rhamnolipids and poly(beta -hydroxyalkanoic acids) (PHAs) are important fermentation products of Pseudomonas aeruginosa. Both contain beta -hydroxyalkanoic acids as main constituents. To investigate the possible relationship between their syntheses, we studied the n-hexadecane fermentation by P. aeruginosa (ATCC 10145). PHA synthesis was found to occur only during active cell growth, while substantial rhamnolipid production began at the onset of the stationary phase. The specific synthesis rate of beta -hydroxyalkanoic acids was estimated as 12.6 mg HA/(g dry cells h) from the PHA formation during the exponential-growth phase. A similar rate was obtained from the beta -hydroxyalkanoic acid incorporation in the rhamnolipids produced during the early stationary phase. A regulatory switch of the flow of beta -hydroxyalkanoic acids from PHA polymerization to rhamnolipid synthesis is clearly indicated to occur when the culture reaches the stationary phase. Five rhamnolipid structures were identified using HPLC-MS. Three are monorhamnolipids, two dirhamnolipids. All have a chain of two beta -hydroxyalkanoic acids. The two major components contain only beta -hydroxydecanoic acids; the three miners also have a beta -hydroxydecanoic acid linked to the sugar but a beta -hydroxydodecanoic acid or beta -hydroxydodecenoic acid as the second acid. The PHA accumulation reached about 7.5% of the cell dry weight. The monomer composition was relatively constant at different stages of production: in weight fractions, beta -hydroxyoctanoic acid, 0.25 (+/-0.05); beta -hydroxydecanoic acid, 0.41 (+/-0.06); beta -hydroxydodecanoic acid, 0.11 (+/-0.05), beta -hydroxytetradecanoic acid, 0.11 (+/-0.06), and beta -hydroxyhexadecanoic acid, 0.12 (+/-0.06). beta -Hydroxydecanoic acid was clearly the primary monomer.