Field trials of fibre flax were sown in 2002 and 2003 comprising 29 and 26 flax varieties, respectively. In 2002, two nitrogen treatments of 40 and 80 kg ha(-1) were imposed. The trials were sprayed with a desiccating herbicide prior to retting at 35 and 15 days after midpoint of flowering in 2002 and 2003, respectively, and were harvested once retting had completed. Fibre was extracted from the flax stems using a laboratory-scale scutcher and hackling pins, and long and total fibre yields were determined. The breaking load of flax technical fibres was assessed using a novel technique employing 'zero twist' yarns. Fineness was assessed using the Wool Industries Research Association airflow method. The tenacity of a single technical fibre was then derived from the average breaking load of the yarn and knowledge of its fineness. Both flax variety and season were found to contribute to variations in fibre fineness and strength, with the dry season in 2003 leading to less variation across the varieties and much finer fibres. Few of the varieties were found to be stable across seasons, and environmental variation accounted for 96% of the variation in fibre fineness between years, and 69% of the variation in tensile strength and breaking load. Fibre yield was found to be related to fibre fineness, with the coarser fibres corresponding to higher yields. The new yarn test presented herein has been shown to be a valid and useful method for determining the tensile properties of technical flax. The varieties and the sample sizes required to adequately measure variation in fibre properties are discussed.