Uncaria guianensis (Aubl.) J.F. Gmel. and Uncaria tomentosa (Willd. ex Roem. & Schult.) DC. (Rubiaceae) have a bark commonly used medicinally by ethnic peoples in South America. Both plants, which are called “cat's claw”, are now being commercialised, but unfortunately they are often sold adulterated with other species. The aim of this work was to determine anatomical features of diagnostic value that could allow the recognition of the bark of both species as raw or processed products. The wood anatomy was also studied. The bark of these two species of Uncaria is very similar, but they differ in the distribution of fibers and the presence of styloids. The wood of both taxa is homogeneous and there are only slight differences in quantitative aspects of conducting elements.