In this paper we describe an in-line single-mode fiber optic wavelength division multiplexer/demultiplexer designed to operate at 1.32 um and 1.55 um. This device was fabricated using evanescent wave coupler technology and consequently exhibits an insertion loss on the order of 0.05 db and a high degree of stability with regard to changes in temperature and input polarization. As a demultiplexer, the device provides wavelength isolation of 16 db at 1.32 um and 18 db at 1.55 um, both of which can be increased to greater than 40 db with the addition of simple edge filters.