The bacterial production of medium-chain-length polyhydroxyalkanoate (PHA) in continuous two-stage cultivation was investigated. Two chemostats were linked in sequence and Pseudomonas putida GPo1 was supplied with different concentrations of octanoic acid in the first and 10-undecenoic acid in the second reactor. PHA isolated from the second bioreactor was a blend of two copolymers typically consisting of poly(3-hydroxyoctanoate-co3-hydroxyhexanoate) (PHO) and poly(3-hydroxy-10-undecenoate-co-3-hydroxy-8-nonenoate-co-3-hydroxy-6-he ptenoate) (PHUE) with structural (monomeric) purities of 85-95 mol %, indicating simultaneous PHA accumulation and degradation inside the cell. The maximum PHA content was found to be 53.8% of total cell dry weight, which is very high for mcl-PHA.