A two-stage aerobic fed-batch process for the biotechnological production of polyesters containing 4-hydroxyvaleric acid (4HV) and medium-chain-length hydroxyalkanoic acids by a recombinant strain of Pseudomonas putida was developed in mineral salts medium with octanoic and levulinic acid as carbon sources at a 15-L scale. The cells were first grown to high densities on octanoic acid at a pH between 7 and 8 and at a temperature of 30 degrees C. Accumulation conditions were induced in the second stage by nitrogen starvation at pH 7.0 and at 35 or 37 degrees C while levulinic acid was continuously supplied. At the end of the accumulation phase, 4HV-containing polyesters, contributing up to 50% (w/w) of the cellular dry weight, were cast into films after extraction with chloroform and precipitation with ethanol, and were spun to fibers. The unprocessed as well as the processed polyesters were characterized with respect to their molecular weight and their thermal, rheological, and mechanical properties. These polyesters showed a distinctly elastomeric behavior resulting from the low content of medium chain-length hydroxyalkanoic acids. The polyester revealed an extremely high elongation at break of approximately 1000%; the molecular weights (M-w) were between 3.3 x 10(5) and 9.4 x 10(5) g/mol and decreased during the melt spinning process.