The solid oxide CaO catalyst derived from natural white bivalve clam shells were employed to produce biodiesel via transesterification of waste frying oil (WFO). The shell materials were calcined in a muffle furnace at 900 degrees C with an optimum time of 4 h to transform calcium species in the shells into active CaO catalyst. The catalyst was characterized using X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and Energy Dispersive Atomic X-ray Spectrometry (EDAX). The activity of the natural CaO catalyst was compared with the commercial CaO. A high biodiesel yield of 95.84% was obtained at a methanol/oil molar ratio of 18:1, 8.0 wt.% catalyst, 3 h reaction time and temperature of 65 degrees C. The biodiesel composition was determined by Gas Chromatography coupled with Mass Spectroscopy (GC-MS). (C) 2013 Elsevier Ltd. All rights reserved.