In his revision of the Psyllids of Zetterstedt (Ossiannilsson, I942), the author identified Chermes viridulus Zett. with Trioza cirsii Löw. By a study of preimaginal characters this is now found to be wrong and it is stated that Trioza viridula (Zett.) and Trioza cirsii Löw are distinct though closely related species. Details of their male and female genitalia and the microsculpture of the under side of their fore wings are reproduced for comparison. Figures of the larval instars of viridula and cirsii and the related agrophila Löw showing their chaetotaxy are also given. A key for the separation of these larvae and some data on the biology and distribution of these species in Sweden terminate the present paper.