Retting is the process of removing pectin (external and internal) from the stem, with the intention of separating the bundles of fibres. The Retting process is based on the different resistance of pectin and cellulose to microorganisms and chemical reagents. Pectic substances are far less resistant to the those kind of effects then cellulose. Depending on the type of process chosen, wastewater has higher or lower amounts of dissolved organic matter. In this paper, we explore the possibility of using ultrasound in flax retting and we then determine the quality of the wastewater after the retting. Three different cultivars were taken for retting, and these were retted in the traditional way and also using ultrasound. After retting and drying with a roller grinder, the woody part of the stem was separated and the yarn was turned into fibre. All investigations were performed on samples of technical flax fibres. The samples were tested for their length, fineness and strength. We then analysed the wastewater where we measured the UV / vis spectra, conductivity, pH, and washed wastewater residues for the two different retting methods. After analysing and processing the data, it is evident that the fibres retted with ultrasound are slightly coarser and weaker, but the wastewater from these methods is environmentally friendly.