In this thesis, agronomic traits were investigated on five varieties of fiber flax. The trial was set up at the Maksimir Experiment Station of the University of Zagreb, Faculty of Agriculture, in 2014 according to the randomized complete block design in four replications. Five European varieties of fiber flax (Viking, Viola, Venica, Agatha and Electra) were involved in the trial. After harvesting the fiber flax, the flax stems were subjected to a biological method of maceration with heated water under controlled conditions with different hardness of water (very soft, medium hard and hard) for three days at 32°C. After retting, the dry stem yield after retting, total fiber yield, share of total fiber, long fiber yield and share of long fibre were determined. Based on the analysis of variance, a significant difference was found for the dry stem yield after retting and the long fiber yield. The variety Viola had the highest dry stem yield after retting. The highest yield of total fiber yield and long fiber and share of long fiber was achieved by the variety Electra. Venica proved to be the best for the share of total fiber. Retting with soft water gave the best results for all the investigated agronomic traits.