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In response to the environmental problems caused by plastics of petrochemical origin, a reduction in the use of these materials and their replacement by biodegradable polymers have been sought. Polyhydroxybutyrate (PHB), a biopolymer of biological origin that belongs to the polyhydroxyalkanoates (PHAs), is similar to polypropylene in terms of its mechanical properties, thermodegradability and melting temperature. Various microorganisms, including cyanobacteria, can synthesize this biopolymer. The objective of this study was to stimulate biopolymer synthesis by Spirulina sp. LEB 18 that was grown under different nutritional conditions. Initially, the growth was conducted with Spirulina sp. LEB 18 without the adaptation of the inoculum. In these assays, the concentrations of the carbon, nitrogen and phosphorus sources were varied. The assay that showed the maximum concentration of biopolymers was reproduced with the adaptation of the inoculum for 45 days. There was an inverse relationship between the cell growth and biopolymer synthesis. The assay that contained 0.25 g L-1 sodium nitrate, 4.4 g L-1 sodium bicarbonate and 0.5 g L-1 potassium phosphate showed the maximum cell concentration (0.6 g L-1) and a low biopolymer accumulation (13.4%). In addition, the assay that contained 0.05 g L-1 sodium nitrate, 8.4 g L-1 sodium bicarbonate and 0.5 g L-1 potassium phosphate produced a high biopolymer concentration (30.7%) and a low cell concentration (0.5 g L-1). The adaptation of the inoculum increased the cell concentration by 7.0% and the biopolymer yield by 20.5%. The biopolymer production was more efficient in assays in which the nitrogen was restricted and had maximum carbon consumption.