Abstract. Title of the paper: Comparative investigations for ultrasonic fat thickness measurements of pigs at the performance testing In two series (serie 1: N = 62 animals; serie 2: N = 26 animals, 22 persons) we have done investigations to specific differences for tools and persons during measurements of fat thickness of living pigs by ultrasonic. Our aim was to explain the causes and the effect of specific differences and to deduce consequences for the performance testing of boars and sows in our breeding companies. In serie 1 there were measured with four ultrasonic tools with different specifications. We find significant differences, which can be up to 4 mm in order of the ultrasonic tool. Especially, measurements with the Lean-Meater, Level 2 suggest a fat thickness, which leads to a clear underestimation of the fat thickness of slaughtered animals. Between the A-Scan-Ultrasonic-tools we can find the following range in the middle fat thickness: Lean-Meater-Level 2 (13.1 mm) < Lean-Meater-Level 3 (13.9 mm) < USN-Tools (15.2 mm) < Piglog105 (Level "Normal") (16.8 mm). All proved tools reflects the degree of fatty degeneration, measured by fat thickness from classification, with a high precision (R > 0.8–0.9).