The values of carbon and nitrogen content, and natural abundance of stable isotopes were measured in mesozooplankton samples from two shelf stations off A Coruña (NW Spain) at monthly frequency during 2000-2001 and 2006-2009. The objective was to compare the composition values of unpreserved with those of preserved samples. Samples were collected approximately at noon with double oblique hauls with Juday-Bogorov or Bongo nets (50 cm diameter, 200 µm mesh-size) from the surface to 50 m depth or near bottom (shallower station). Each sample was divided with a Folsom splitter in two aliquots: one was dried immediately (50°C, 48 h) and the other was preserved in formalin (borate-buffered formaldehyde, 4% final concentration). The dried samples were grinded and analysed using an isotope-ratio mass spectrometer coupled to an elemental analyser within one year of collection. The preserved samples were stored for up to 10 years before analysis as with the unpreserved samples. In addition the contributions of copepods to total dry weight biomass was estimated from species abundance values determined by microscopical examination and the values and factors from the literature (Wiebe, 1988; Bode et al., 2012; Acuña et al., 2014).