This dataset contains the values of standard length, wet weight, and body mass-density of Cyclothone spp. measured during the SUMMER cruise (from 30th September to 18th October 2020). Samples were obtained with the "Mesopelagos" midwater trawl during day and night hauls carried out from the surface to 700-800 m depth. The Mesopelagos net consists of graded-mesh netting starting with 30 mm and ending with 4 mm. The specimens collected were immediately identified and measured (standard length, SL). Body-mass densities (ρ) of Cyclothone braueri, C. pseudopallida, C. pygmaea and C. pallida were determined on board via a density-bottle method (Greenlaw, 1977; Mikami et al., 2000).