Ammonoid soft tissues are rarely preserved and such findings are of considerable interest. Extremely little is known about the structure of ammonoid siphuncle soft-parts. Until now indisputable fossilized siphuncle soft tissues were known only in a few ammonoid genera. This article describes phosphatized soft tissues inside the siphuncle tubes of the Upper Jurassic (upper Volgian) ammonite Kachpurites fulgens from Central Russia. Cross-sectional views of several Kachpurites siphuncles show a large blood vessel in the center surrounded by four smaller ones. In several specimens, the siphuncle epithelium is preserved. The size and arrangement of the blood vessels in Kachpurites is nearly identical to previously described blood vessels in the siphuncles of Recent Nautilus and the Permian prolecanitid ammonoid Akmilleria electraensis. This similarity indicates a remarkable conservatism of ammonoid siphuncles and provides grounds for the assumption that the last common ancestors of Ammonoidea and Nautilida also had the same structure of the siphuncle soft tissues.