The regular sea urchin Lytechinus variegatus, a species previously reported from areas of reduced salinities,and Arbacia lixula, a species unreported from diluted waters, were submitted to seawater dilution orseawater dilution in magnesium-supplemented waters. Seawater (35 psu) was either proportionallydiluted with filtered dechlorinated tap water (30 psu, 25 psu), or diluted and supplemented with magnesiumas MgCl2 (30+Mg, 25+Mg), up to full-strength seawater Mg2+ levels (35 psu, ~54 mM Mg2+). Magnesiumsupplementation was intended to verify the interfering effect of magnesium on osmo-ionic concentrations ofthe coelomic fluid (CF) of two ecologically distinct species of sea urchins. After 6 h in control (35 psu) orexperimental seawater, CF samples were withdrawn by puncturing through the peristomial membrane.Coelomic fluid osmolality ([Osm]), and concentrations of ([Na+]), ([Cl-]), ([Mg2+]) and ([K+]) weremeasured for both species. Under all conditions, L. variegatus displayed higher CF osmolality, [Na+], and[K+] values than the water (and A. lixula). Comparatively, L. variegatus is designated as a‘hyper-conformer’, while A. lixula is an ‘iso-conformer’. The CF [Mg2+] showed no evidence of being controlled by either species. Mg2+ supplementation in diluted seawater affected Mg2+ and Cl- levels only. Na+ appears to be taken up actively by L. variegatus, rendering its CF mostly hyper-ionic for Na+ (and hyperosmotic) relative to external seawater. The different gradients observed with the different ions suggest selective permeabilities orion regulation by L. variegatus.