Peritoneal mesotheliomas in children are of rare occurrance. We herein report the clinical, radiological, and pathological findings of a rare case of malignant peritoneal mesothelioma occurring in nine-year-old female child. The child presented with abdominal distension and awareness of a painless mass in the abdomen which on radiology appeared as a large heterogeneous pelvic mass with peritoneal deposits at multiple sites. To the best of our knowledge, this is the first case of a peritoneal malignant mesothelioma on which fine needle aspiration (FNA) was performed as first line investigation of the primary tumor. The cytological features, major differential diagnoses, and the pitfalls therein are discussed. Histopathology revealed biphasic pattern of mesothelioma which is again a rare pattern. Immunochemistry was carried out on the cell block made from the FNA as well as the biopsy specimen essentially showed the same features. There was positivity for vimentin, EMA, and cytokeratin 5/6 while WT1, calretinin, and CEA were negative; however, D2-40 showed diffuse membranous positivity in the epithelial areas and cytoplasmic positivity in the spindle areas confirming a mesothelioma. We emphasize the use of immunochemistry on cell block material for a confident diagnosis of mesothelioma in such cases. Diagn. Cytopathol. 2012. © 2011 Wiley Periodicals, Inc.