A device has been developed for the measurement of copper(II) ions (Cu++) in aqueous medium. The device reported here is an electrochemical transistor that consists of two platinum electrodes separated by 100-mum spacing and bridged with an anodically grown polycarbazole film. The undoped polycarbazole film is observed to be highly selective for the Cu(II) ions. In a completed device, the conductivity of the polycarbazole film changes on addition of Cu (II)ions. This change in conductivity is attributed to the conformational changes in the polymer phase on occupation of the Cu(II) ions, without affecting electron/proton transfer. The device turns on by adding 2.5 x 10(-6) M Cu(II) ions and reaches a saturation region above a concentration of 10(-4) M Cu(II) ions. In this concentration range, the plot of I-D vs log[Cu(II)] is linear. The selectivity of the device for other metal ions such as Cu(I), Co(II), Fe(Il), Fe(Ill), Zn(II), and Pb(II) is also studied.