Time series length-frequency data are presented for Themisto amphipods collected as swimmers by moored sediment traps since 2000 at the AWI deep-sea observatory HAUSGARTEN (79°N/4°E) in the eastern Fram Strait. Amphipod occurrences increased significantly from 2000 to 2009 at 200-300 m depth, and the North Atlantic species Themisto compressa was continuously present in the samples starting in 2004. We present year-round records of large adult Themisto amphipods, including the appearance of T. libellula with a total body length of up to 56.7 mm and juveniles starting from 4.0 mm. The length of T. abyssorum ranged from 4.2-25.6 mm, whereas it varied for T. compressa from 8.8-24.4 mm. Length-frequency analysis indicated a life span of 2 years for T. abyssorum and at least 3 years for T. libellula. The absence of juveniles for T. compressa suggested its reproduction in southern subarctic areas and its occasional northward migration with warmer Atlantic water into the eastern Fram Strait. The seasonal and long-term size structure of the three pelagic species was consistent over the course of the study, indicating no changes occurred in cohort development due to increasing abundances or warming water temperatures.