With the exception of isolates from blood and homogenates of diseased fish, the presence of Egtved virus (=VHS, viral haemorrhagic septicemia of the rainbow trout) could not yet be demonstrated directly in host organs by electron microscopy, although recently, antigen has been identified in a number of organs by means of fluorescent microscopy. Demonstration of virus from organ extracts by means of a permanent cell culture from a coldblooded animal: Salmo gairdneri (RTG-2) as described in the present paper does not allow for a clear conclusion as to the presence of virus within organ cells. Egtved virus was studied in various organs (brain, heart, liver, kidneys, spleen, gills) of naturally and artificially infected trouts. Virus could not be demonstrated by electron microscopy in any of the organ sections. However, in RTG-2 cell cultures infected by organ extracts from naturally diseased trouts, a clear cytopathogenic effect could be observed in the 1st passage at + 15 degrees C after 48-72 hours and virus demonstrated by electron microscopy (Table, Figs. 1, 2, 3, 4). Both in organ sections (heart, kidney, spleen, and gills) from diseased trout and in RTG-2 cells infected with organ extracts, additional organisms were observed which on account of their ultrastructure could be classified as rickettsia (Table, Figs. 5, 6, 7, 8, 9). On account of the fact that these organisms were found in the organs of trout infected with Egtved virus and which partially died in great numbers, a pathogenic behaviour of these organisms is suggested. It has not yet been clarified whether they have any influence on the course of the viral infection or temporary mass dying of rainbow trout when kept in hatcheries.