This paper deals primarily with a full redescription of the mature female of Ascidicola rosea Thorell, 1859, and a young female is described mainly in regard to the urosome. The two species of solitary ascidians containing this copepod, which lacks an eye, were collected at different localities: Microcosmus sabatieri Roule at Banyuls-sur-Mer, and Ascidia mentula Müller at Strangford Lough. Many systematists have studied this long-known, elongated, pinkish copepod (Ascidicolidae: Ascidicolinae). However, the major articulation between the metasome and urosome has been interpreted differently by authors, and morphological details have not always been consistently described. The intent of the present paper is to resolve such problems. It shows that the ventral transverse spinose pad posterior to the penultimate (second abdominal) segment of the urosome of the mature female is homologous to much of the ventral part of the penultimate (third abdominal) segment of the urosome of the young female.