All parasitoid apostome ciliates infecting krill in the northeastern Pacific are currently assigned to the genus Pseudocollinia. Each krill specimen is apparently infected by only 1 Pseudocollinia species. We describe Pseudocollinia similis sp. nov., discovered infecting the krill Thysanoessa spinifera off Oregon, USA. Its protomite-tomite stage resembles that of P. beringensis, which infects T. inermis (type host species), T. longipes, and T. raschii females in the Bering Sea. These ciliates have similar numbers of somatic kineties (18-21 vs. 16-20) and typically have 3 oral kineties. Furthermore, these 2 apostomes are sister species on gene trees based on sequences of small subunit rRNA (0.06% difference) and cytochrome c oxidase subunit 1 (cox1; 30% difference). P. brintoni and P. oregonensis are closely related as a separate group from P. similis and P. beringensis. The similar tree topologies based on the cox1 sequences of 21 host krill individuals representing 6 krill species (Euphausia pacifica, Nyctiphanes simplex, T. inermis, T. longipes, T. raschii, and T. spinifera) and the apostomes isolated from these krill suggest host-parasitoid codiversification. However, this hypothesis was statistically rejected by an approximately unbiased test in which the host tree topology was used to model parasitoid evolution (p ≤ 0.05).