Imaging the birefringence characteristics of the retinal nerve fiber layer (RNFL) may aid in glaucoma diagnosis. Polarization Sensitive Optical Coherence Tomography (PSOCT) was used to record in vivo high resolution images of the RNFL in two cynomologous monkeys. Depth variation in the Stokes vector of reflected light was used to calculate the optical phase retardation per unit depth (OPR/UD) as a function of retinal position. OPR/UD decreased from 32/spl deg//100 /spl mu/m near the optic nerve to 5/100 /spl mu/m at a location 600 /spl mu/m nasal to the optic nerve. Variation of OPR/UD in the RNFL with retinal position demonstrates a chance in birefringence for different densities of ganglion cell axons. PSOCT may be useful for noninvasive determination of RNFL thickness and fiber density.