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Medium chain‐length polyhydroxyalkanoates (mcl‐PHAs) are efficiently synthesized from fatty acids. It was hypothesized that under non‐axenic conditions, the anti‐microbial properties of fatty acids would reduce growth of microbial competitors and allow Pseudomonas putida LS46 to dominate the population resulting in non‐sterile mcl‐PHA production. Non‐sterile batch and fed‐batch cultures produced similar final mcl‐PHA content, monomer composition, and productivity during 24–27 h cultivations when compared to sterile control cultures for initial octanoic acid concentrations ranging from 5 to 40 mM at an initial pH of 6.5. In the absence of a P. putida inoculum, growth was eventually observed in non‐sterile medium after a lag period of up to 120 h, the length of which was dependent on the octanoic acid concentration. The efficacy of prolonged non‐sterile cultivation was tested using a sequential batch reactor (SBR). After 28 days (672 h), 1.62 g L−1 of total biomass was observed, of which the mcl‐PHA content was 47%. This resulted in a PHA titer of 0.77 g L−1, and a volumetric PHA productivity of 32 mg L−1 h−1. The polymer composition remained stable throughout at 87.0 ± 2.7% C8 monomers, and short‐chain length (scl‐) monomers were not detected. This study is the first of its kind to report that purely mcl‐PHAs were produced after prolonged periods in a non‐sterile environment and demonstrate that medium chain‐length fatty acids exert a strong selective pressure toward organisms that synthesize mcl‐PHA. This suggests an opportunity for mcl‐PHA production in open continuous cultivation, which could reduce both fixed and operating costs. [ABSTRACT FROM AUTHOR]