We examined the diel vertical migrations of the macroscopic pelagic relict crustaceans Gammaracanthus lacustris, Mysis relicta, Monoporeia affinis and Pallaseopsis quadrispinosa in the deep Lake Paasivesi basin of the Saimaa lake complex. The samples were collected in October 2006 and 2007 using a Hydro-Bios Multi Plankton Sampler onboard the research vessel R/V Muikku. The study provides information about the day-and-night and year-to-year changes in the vertical distribution of these crustaceans. Regarding G. lacustris we explored if body size (length), sex or maturity have bearing on the individuals' depth preference or possible diel migrations. We noticed that G. lacustris females occur, with very few exceptions, in the deepest waters at 50—60 m, while the individuals occuring in the depth range of 25—45 m are almost exclusively males. Our results did not indicate any diel vertical migration behaviour in G. lacustris. In contrast, 24 % of M. relicta were observed to ascend to the depths of 1—20 m during the night time. The migrant M. relicta are mainly young individuals, less than 14 mm long. A night-time ascent pattern, on the average by about 10 metres, was also observed in M. affinis and P. quadrispinosa.