Arctic charr Salvelinus alpinus L. is an important species in Icelandic aquaculture and the most common wild salmonid in Iceland. A study on the course of infection with the bacterium Renibacterium salmoninarum was conducted using 3 different challenge methods in brackish and fresh water. Bacterial isolation, ELISA and PCR tests were used for detection of the bacterium in multiple organ samples. In an experiment, run for 34 wk in brackish water, infection was established by intraperitoneal injection with 5 x 10(6) colony forming units (CFU) fish(-1). There were external and internal symptoms of bacterial kidney disease (BKD) and mortalities between 6 and 13 wk after injection. A cohabitation trial was run simultaneously and infection was well established after 4 wk, as demonstrated by the detection methods applied. Symptoms of BKD were not seen and all but 1 cohabitant survived. In a separate experiment, infection was established by pumping a fixed amount of water from a tank with fingerlings infected by intraperitoneal injection into tanks with naive fish, in fresh or brackish water, for 6 wk. Fish in the inflow tanks were reared for an additional 3 wk. There were neither macroscopic symptoms nor mortalities. ELISA and PCR tests showed that infection started to take hold after 3 wk. The challenge trials demonstrated that Arctic charr is susceptible to R. salmoninarum. Cohabitation and inflow of water from tanks with infected fish provide useful models for further studies on R. salmoninarum infection acquired in a natural way in Arctic charr.