Size, weight, body-mass density and swim bladder measurements of Cyclothone species were analysed during the IDEADOS (Mediterranean Sea, July 2010), Bathypelagic (North Atlantic, May-June 2018) and SUMMER (Mediterranean Sea and North Atlantic, September-October 2020), and cruises. Specimens used to analyse size and weight were collected with a Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS-1 m²), fitted with nets of 0.2 mm mesh size. Hauls were carried out between the surface and 700-800 m, in IDEADOS and SUMMER cruises, and up to 1800 m in Bathypelagic survey. Measures of body mass-density were obtained during the SUMMER cruise from specimens collected using the "Mesopelagos" midwater trawl. Finally, measures of swim bladder length, height, width, angle, volume, equivalent spherical radius, and aspect ratio were measured from specimens collected during the Bathypelagic cruise.