After stimulation in a hypotonic solution (9.4 mOsm kg−1), inseminated eggs of the chum salmon Oncorhynchus keta initiate cleavages in isotonic salmon Ringer's solution (267.3 mOsm kg−1) containing 3.2 mM Ca2+ ions. Blastomeres of these eggs, however, separate from each other and the enveloping layer is not observed at the blastula stage. An increase in external divalent cations rescues the separation; the concentration of CaCl2 in the external medium should be 25 mM or more to induce close contact of blastomeres and the formation of an enveloping layer in isotonic salt solutions. The effectiveness of Ca2+ ions can be substituted by Mg2+, Sr2+ and Zn2+ ions; the same results are obtained in isotonic MgCl2 and SrCl2 solutions (100 mM) or in isotonic salmon Ringer's solution containing Zn ions (6.2 mM). The close contact of blastomeres and the formation of an enveloping layer are also observed in a low Ca2+ concentration (< 0.1 mM) in a hypotonic salt solution (9.4 mOsm kg−1). The Ca2+ level in the external medium to induce the enveloping layer formation seems to be correlated with the salinity of the incubation medium. It is suggested that adhesion molecules on the surface of blastomeres in the chum salmon eggs are different in properties from those found in sea urchin and other fish species.