Flax (Linum usitatissimum L.) is an annual plant species widely cultivated in temperate climates for bast fibres and linseed oil. Apart from traditional textile use, the fibres are fast becoming an integral part of new composite materials utilized in automobile and constructive industry. Especially attractive for environmental safety demands are biodegradable and renewable biocomposities based on polyhydroxybutyrate (PHB) polymer as a matrix and reinforced with the flax fibres. Manufacturing of PHB by bacteria fermentation is however substantially more expansive as compared to technologies producing conventional plastics. We report for the first time generation of transgenic plants which produce both components of flax/PHB composites, i.e. the fibres and the thermoplastic matrix in the same plant organ of a crop. The flax (cv. Nike) plants were transformed using constructs bearing either single cDNA, encoding the beta-ketothiolase enzyme (C plants), or all three of the genes necessary for poly-beta-hydroxybutyrate (PHB) synthesis (M plants). Both constructs contained a plastidial targeting sequence. The amount of PHB produced by the transgenic plants was up to over 70-fold higher than in wild-type plants, when analysed using the gas chromatography/mass spectrometry (GC-MS method). The PHB accumulation in plastids caused change both in their shape and size. The use of a stem-specific promoter for transgene expression protected the transgenic plant from growth retardation and also provided higher PHB synthesis than in the case of constructs governed by the 35S CaMV constitutive promoter. None toxic effects that could lead to stunted growth or the loss of fertility were observed, when 14-3-3 promoter was used as the stem-specific. Significant modifications in stem mechanical properties were accompanied to the PHB accumulation in growing cell of fibres in the transgenic plants. The Young's modulus E, the average measure of stem tissues resistance to tensile loads increased up to twice in M plants as compared to a single gene transformed ones. However, a wide range of E values, from 24.1 to 54.4 MPa, was observed in dependence of tested strain. Potential commercial significance of the genetic manipulation approach enabling synthesis of thermoplastic in crops cultivated for fibres is discussed.