Rhizocephala is a group of unique and highly specialized parasitic crustaceans. Due to specialization in endoparasitism adult rhizocephalans have lost almost all the traits of free-living crustaceans. Despite large amount of data on different aspects of their biology there is still insufficient knowledge of their interna morphology. The muscular system in interna has been found and described only for Peltogaster paguri from the family Peltogastridae. The aim of our study is to visualize and describe muscular system in the interna of Polyascus polygenea and Sacculina pilosella (family Sacculinidae). Representatives of both species have similar type of muscular system, which is significantly different from that in P. paguri. The muscular system of the sacculinids consists of muscular rosettes located within the body wall of each rootlet. Muscle fibers in these rosettes are oriented in different directions. Tiny muscular processes connect the rosettes to each other. These contractile elements form a joint net-like muscular system. We suggest that this muscular system provides peristaltic movement of the roots and transports nutrients within the interna.