The authors report a case of low-grade retroperitoneal malignant peripheral nerve sheath tumor (MPNST) showing Schwannian and fibroblastic differentiation in individual tumor cells. The tumor was detected in a 29-year-old male and posed diagnostic difficulty because of the unusual morphologic and immunophenotypic features. Morphologic examination of the H&E sections revealed a rather circumscribed, highly vascular, moderately cellular spindle cell tumor. The neoplastic cells were arranged in vague, short fascicles, distributed haphazardly amid hemangiopericytoma-like vascular channels, and showed occasional whorls. Myxoid stroma and keloid-like collagen bundles were frequently seen. There were satellite nodules outside the main tumor mass and low mitotic activity but no necrosis. The tumor cells stained strongly and diffusely for both S-100 protein and CD34. Electron microscopy revealed cells with processes and focal lamina, and prominent rough endoplasmic reticulum. Although the capacity of MPNST to exhibit divergent differentiation is well known, fibroblastic differentiation is generally poorly and inconsistently documented. The present case represents an unambiguous demonstration of the co-expression within individual tumor cells of Schwannian and fibroblastic differentiation in a low-grade MPNST. The literature on this subject is reviewed.