There are few data on the effect of long-term manipulations on soil protozoa, and almost nothing is known about soil protozoa in Alaska. I studied the response of testate amoebae to nitrogen and phosphorus addition in an Arctic fen, at Toolik Lake Long-Term Experimental Research (LTER) Station, Alaska. Testate amoebae were extracted from Sphagnum mosses in control and fertilized plots. Of the 35 testate amoebae taxa recorded, 7 are first observations for the Arctic (excluding Russia) and 14 for Alaska. The total density and biomass of testate amoebae were significantly reduced, by 77% and 84%, in the fertilized plots. The structure of testate amoebae communities was also modified in those plots, although for most taxa the changes were not significant. Four taxa (Amphitrema flavum, Assulina muscorum, Placocista spinosa ssp. hyalina, and Hyalosphenia papilio) accounted for over half of the population in the control plots but only for 11% in the N and P plots. The densities of A. muscorum and Difflugia oviformis were significantly lower in the N and P–treated plots. The relative abundance and contribution to biomass of Centropyxis aerophila, Phryganella acropodia, and Tracheleuglypha dentata increased in the fertilized plots, while that of D. oviformis decreased. These effects suggest that testate amoebae respond to nutrient manipulations in the Arctic.