Anaplasma phagocytophilum transmitted by Ixodes spp. ticks is the causative agent of tick-borne fever (TBF) in domestic ruminants. TBF is widespread along the coast of southern Norway and may cause a severe problem for the sheep industry. Red deer (Cervus elaphus) are important hosts for ticks and have been found to be infected naturally with A. phagocytophilum. However, it is unclear whether red deer could serve as reservoir hosts for A. phagocytophilum infections in sheep. We infected lambs experimentally with a red deer and a sheep isolate, respectively. The 497 base pairs of the partial 16S rRNA gene sequences of both isolates were 100% identical to GenBank accession number M73220; the 3.8 kilobases of the total ank gene sequences were 99% identical. Sixteen lambs were used, four lambs in each group. Two groups were inoculated with the red deer isolate on day 0, and then challenged on day 42 with the ovine or the red deer isolate, respectively. The third group was inoculated with the sheep isolate on day 0 and challenged with the red deer strain on day 42. Four lambs were used as uninfected controls. Blood samples for hematology, bacteriology, and serology were collected regularly for 12 wk. Presence of A. phagocytophilum in blood was determined using blood smears. Serologic response was measured by indirect immunofluorescence. Although animals inoculated with the ovine strain showed more severe clinical manifestations, lambs infected with the red deer isolate reacted with typical signs of TBF such as fever, bacteremia, and neutropenia. We conclude that A. phagocytophilum strains causing TBF in sheep might circulate in the red deer population in Norway.