The aims of this study were to determine the prevalence of fin damage in farmed rainbow trout and to see whether the level of damage differed between different fish categories and farms. The study was field based and included the fin damage analysis and clinical description of the damaged fins. Fins were analyzed in two categories of fish (weight below 30g (min. 5g) and over 100g (max. 250g)). Thirty fish per category were randomly selected, netted and each rayed fin was assessed and photographed (total of 5880 fins were analyzed in 840 fish from seven rainbow trout farms). The prevalence of fin damage was 100% and there was a large range in the level of damage which was mainly characterized by surface abrasions. Worst affected fins in both fish categories were dorsal and pectoral fins. Fin damage was present to a lesser degree in the smaller categories, but there was fin damage in the smallest fish examined. Pattern of damage was Dorsal > Pectoral > Abdominal > Anal > Tail fin. Differences in fin damage in all surveyed farms indicate that some factor or group of factors specific to each farm influence the extent of damage. Fin damage is operational welfare indicator and future research should identify and explore the impact of the factors affecting fin damage and propose management practices that can minimize the level of fin damage. Additional knowledge is needed to identify whether fin damage is etiologically connected to different production system, handling procedures or another background.