Yeast aconitase [citrate (isocitrate) hydro-lyase, EC. 4. 2.1. 3], inductively formed by Candida iipolytica in the presence of fluoroacetate, was purified approximately 100-fold by Sephadex G-100 gel filtration and DEAE-Sephadex column chromatography, yielding dark-brown needle crystals. The crystalline aconitase was homogeneous as judged by polyacrylamide gel electrophoresis and sedimentation by ultracentrifuga-tion. The enzyme showed maximal activity at pH 8.0 and at 55°. It has an S20.w of 5.03 S, a molecular weight of 68,500 and an isoelectric point of pH 4.2. The presence of 2.10 moles of iron per mole of the enzyme was demonstrated by atomic absorption spectroscopy.