Bonaccordite, Ni2FeBO5, is a very rare mineral named after the Bon Accord area, Barberton, Transvaal, in South Africa, where it was discovered in 1974 [1,2]. It is a nickel boroferrite, and an analog of more common and known ludwigite, Mg2FeBO5 [3,4]. Recently, bonaccordite particles have been discovered in corrosion product deposits (crud) on fuel pins in some of pressurized power reactors (PWRs) experiencing Axial Offset Anomaly of power and neutron fluxes [5,6]. Mossbauer spectroscopy and X-ray diffraction showed that such fuel crud samples contain—in addition to nickel oxide, nickel ferrite and zirconium oxide—a considerable amount of Ni2FeBO5 usually in a form of slender rod-shaped particles (needles) !10 lm long and !0.1 lm thick. It was also shown that bonaccordite can be synthesized hydrothermally in lithiated water, near or in supercritical state [7]. In the present work the crystal structure of the rod-like particles in the crud samples extracted from the surface of fuel rods in a PWR core and examined previously with other methods [3] was investigated using electron diffraction. The purpose of the electron diffraction examination was to determine whether the individual bonaccordite needles formed in the reactor have welldefined crystalline structure, and to check whether indexing of their electron diffraction patterns agrees with known space group and unit cell parameters of bonaccordite.