A flexible, intracochlear multimicroelectrode array is being developed for differential electrical stimulation of the cochlear nerve in human sensory deafness [M. Sonn, Proc. 25th Annual Conf. Eng. Med. Biol. 14, 159 (1972)]. The conductor elements, 0.3 μm thick, are formed on both sides of a thin-film plastic substrate using vacuum deposition and photolithographic etching techniques. In the present configuration, the array consists of 33 platinum conductors, 12.7 μm wide, each terminating at an electrode surface, 250 μm wide. The electrodes are spaced on 700 μm centers. Substrates investigated include Mylar, Parylene-C, and FEP and TFE Teflon films, 50 to 130 μm thick. Of these, Mylar was found best for electrode fabrication. Insulation of the array is accomplished by dip coating in Silastic. The electrode tips are exposed by mechanical scribing techniques. Short-term body tolerance studies in guinea pigs show good compatibility with the electrode materials. Bipolar sinusoidal stimulation of the electrodes in normal saline shows a decrease in complex impedance proportional to the −0.99 power of the frequency: 23.8 MΩ at 100 Hz to 250 kΩ at 10 kHz. Stimulation studies in vitro and in vivo (scalae tympani of cats) are planned to determine behavioral responses to electrical stimuli and long-term tissue reaction and electrolytic dissolution effects. [This work was supported in part by Raytheon Company.]