The northernmost self-sustaining populations of European lobster (Homarus gammarus) are found north of the Arctic Circle (66°33'N), in Tysfjord and Nordfolda, two large, deep, and relatively warm fjord systems in northern Norway. In contrast to most other Norwegian lobster populations, these genetically distinct populations are still found at relative high densities. This paper presents results from surveys conducted in the fjord systems in 1964-65 and between 1992 to 2005, and compares reproductive characteristics, morphology, growth, and catch-per-unit-effort (CPUE). The lobster population in Stefjord, one of four individual fjords in the Tysfjord fjord system, was characterised by a small mean carapace length (CL, 78 ± 11 mm SD), and less than 18% of the catches were larger than the minimum legal size of 88 mm CL. Growth rates were also low, with an average increment per moult of 4.0 ± 0.9 mm CL for females and 5.6 ± 2.1 mm for males. The crusher claw increased on average 5.5 ± 2.3 mm at each moult in females, and 11.2 ± 7.6 mm in males. The smallest ovigerous female was 62 mm CL, and more than 60% of females above 66 mm CL were berried (egg-bearing). In contrast, lobster in the Nordfolda fjord system were larger, with more than 75% of the catch larger than minimum legal size, and the smallest ovigerous female was 76 mm CL. Males in Nordfolda increased on average 8.3 ± 2.4 mm in CL at each moult. Lobster populations in the area of the midnight sun (24-h daylight during summer) were also morphologically different from lobster off southwestern Norway in crusher claw length for males and abdominal width for females. There were no differences in the size distribution or CPUE between the surveys in 1964-65 and between 1992 and 2005, leading to the conclusion that population density seemed to be relatively stable. The average density in the fjord systems of Tysfjord and Nordfolda was 155 ± 76 lobsters and average weight was 71 ± 26 kg per km shoreline in the main survey areas. The adaptation to a sub-arctic environment has resulted in geneticlly distinct populations as well as slightly altered biology and behaviour. Our assessment suggests that the populations are vulnerable to exploitation, and conservation plans should be elaborated. © The Royal Society of New Zealand 2009.