Renibacterium salmoninarum (Rs) is common in wild Arctic charr, Salvelinus alpinus, and brown trout, Salmo trutta, in Iceland. Of 22 charr and nine trout populations, none were free of Rs antigens. In two charr populations only one fish exceeded the Rs antigen detection limit and in one of these cases the ELISA value was within uncertainty limits of the infection criterion. Mean prevalence of infection was 46% for Arctic charr (range: 3-100%) and 35% for brown trout (range: 6-81%). No infected fish showed gross pathological signs of bacterial kidney disease (BKD). The ubiquity and high prevalence of infection indicated that the bacterium has been endemic for a long time, and is probably a normal, low density resident in the fish. A lack of correlation in mean intensity of Rs antigen and prevalence of infection between sympatric populations of Arctic charr and brown trout suggests that the dynamics of infection and internal proliferation of bacteria can be quite independent in the two species even if they live in the same lake. Rs intensity and its coefficient of variation decreased with age in older fish, suggesting a connection between Rs intensity and host mortality. However, this can be caused by other ecological factors that decrease survival, especially low food availability, which simultaneously increase the susceptibility of Rs infection and internal proliferation.