The female and male of Mychophilus roseus Hesse, 1865 (Enteropsinae) are fully redescribed based on specimens living in the compound ascidian Botryllus schlosseri (Pallas, 1766) from Roscoff. The female has three submarginal setulose setae on the labrum; the labral setae of M. roseus have not been reported previously. The anus opens as a transverse slit near the dorsal distal margin of the leg 4-bearing segment. The presence of labral setae (number varies) and the displacement of the anus (transverse slit) to the dorsal side (location similar) are probably diagnostic features for females of the genus Mychophilus. With respect to the rostrum and many of the appendages, the male of M. roseus is clearly distinguished from the male of Enterocola fulgens van Beneden (Enterocolinae).