Cleavage of the embryo of Halocynthia roretzi was arrested with cytochalasin B at 1- to 32-cell stages and the embryo was cultured in sea water containing cytochalsin B until a developmental time equivalent to the hatching of the control larva. Membrane properties of the blastomeres were studied with constant-current and voltage-clamp techniques. Four types of membrane response - neural, epidermal, muscular and non-excitable - were identified on the basis of the shapes and ionic dependence of action potentials in the blastomeres of 8- to 32-cell embryos. Only the epidermal type of response was found in the blastomeres of 1- to 4-cell embryos. The blastomeres with responses of neural type had Na, Ca, delayed K rectifier, anomalous K rectifier and Ca-induced K channels. Those of epidermal type had Ca, anomalous K rectifier and Ca-induced K channels. Those of muscular type had Ca, delayed K rectifier, anomalous K rectifier and possibly Ca-induced K channels. Those of non-excitable type had almost none or small amounts of outward- and inward-going rectifier channels. The characteristic responses of neural type were found in small blastomeres in the animal hemisphere, which included some presumptive neural regions. The responses of muscular type were found in large blastomeres of the vegetal hemisphere, which included some presumptive regions for muscle. Those of epidermal type were found in the blastomeres of the animal hemisphere which did not differentiate into the neural type. Those of non-excitable type were found in some blastomeres of the vegetal hemisphere. Blasomeres of 1- to 32-cell cleavage-arrested embryos, which were presumed to possess more than one possible developmental fate, did not develop mosaic membrane properties but differentiated into one of the four types, with a probability dependent upon a gradient of ooplasmic segregation at the time of arrest.