Although widespread throughout the continental United States, Myxobolus cerebralis, the myxozoan parasite that causes whirling disease in salmonids, has not been reported from the state of Alaska. As part of a risk assessment for the introduction and establishment of M. cerebralis into Alaska, the distribution of the invertebrate host Tubifex tubifex was surveyed, and its genetic composition and susceptibility to the parasite were determined. Many oligochaetes, but no T. tubifex, were collected from southeastern Alaska; however, 4 lineages of T. tubifex (I, III, IV, and VI) were identified from south-central Alaska. Lineage IV had not been previously described in North America, and its susceptibility to M. cerebralis was unknown. When lineage IV T. tubifex and 3 mixed lineage (I, III, IV, and VI) groups were exposed to M. cerebralis, only lineage III became infected under our experimental conditions. Infection occurred in this lineage even when it made up just 3% of the population. Implications of the presence of nonsusceptible lineages of T. tubifex on Alaskan salmonids would be significant in areas where these lineages dominate T. tubifex populations.