The adhesive properties of the coelomocytes of the sea cucumber,Holothuria polii, have been investigated by studying their ability to attach to glass coverslipsin vitro, and their morphology examined by scanning electron microscopy. Both amoeboyctes and spherule cells in cell suspensions attached themselves to glass coverslips, but spreading activity was restricted to an amoebocyte subset which assumed an extremely flattened morphology. Coelomocyte adhesion was a time- and temperature-dependent phenomenon and required cations for attachment to the glass surface