Parasites transmitted through the consumption of infected prey items are commonly used to examine patterns of host feeding. However, an estimate of the prevalence of larval parasites in consumed prey items is often lacking restricting the ability to translate the number of parasites observed into the number of prey consumed. This study examined the prevalence of the nematode parasite Hysterothylacium aduncum in the benthic amphipod Americorophium salmonis recovered from the stomachs of juvenile Chinook salmon (Oncorhynchus tshawytscha) collected near the mouth of the Columbia River estuary (USA) in May, June, and July of 2011. This study represents the first estimate of the prevalence of H. aduncum in A. salmonis from this locality. The stomachs of 118 subyearling Chinook salmon were examined for prey items. We recovered 472 undigested A. salmonis from stomachs that were then examined for juvenile H. aduncum. Six of the 472 A. salmonis were infected with H. aduncum (1.3%). However, prevalence and mean intensity of H. aduncum was 74.6% and 12.9 ± 15.0 S.D. respectively in the gastrointestinal tracts of those salmon. The low prevalence of the parasite in the ingested A. salmonis and the high recovery of adult parasites in subyearling salmon suggest salmon fed on perhaps hundreds of this prey item to acquire the observed numbers of H. aduncum as opposed to the 4.0 ± 6.2 S.D. A. salmonis found on average during stomach content examination. This reemphasizes the importance of A. salmonis as a prey item for subyearling Chinook salmon within the estuary.