Abstract: Apparatus for visually examining the fuel rods of an assembly of fuel rods immersed in the tank of a nuclear reactor or in a deactivation tank, comprises an endoscope including a prism and an optical system of stabilized glass, and at least one arm supporting a mirror facing the prism and inclined relative to the plane defined by the axis of the tube of the endoscope and the arm, the thickness of the arm-mirror assembly being less than the free space between the rods of the assembly. In use the apparatus is mounted on a system of carriages which enable the apparatus to move in three perpendicular directions, the carriages being associated with rotatable means for supporting an assembly of rods to be examined.