Salmincola californiensis is a parasitic copepod that infects salmonid fishes in the genus Oncorhynchus. Our objective was to determine the level of infection of S. californiensis in Rainbow Trout in Birch Creek, Idaho. We sampled Rainbow Trout at 5 sites on Birch Creek during October 2015 and May 2016. Mean intensity of infection was greater in May 2016 (x̄ = 1.7) than it was in October 2015 (x̄ = 2.9; F1, 69 = 5.535, P = 0.021). Intensity of infection ranged from 1 to 10 copepods in October 2015 and 1 to 12 copepods in May 2016. Intensity of infection did not differ among sites (F4, 69 = 0.141, P = 0.966) or by total length of the fish (F1, 69 = 3.036, P = 0.086). We found that the odds of infection of Rainbow Trout increased by 3% (95% CI, 2.1% to 3.9%) for every 1 mm increase in total length (χ20.05, 321 = 59.469, P < 0.001). Site 5 had the highest odds of infection (χ20.05, 317 = 11.656, P = 0.020). In addition, Rainbow Trout were 2.3 times (95% CI, 1.28 to 4.33) more likely to be infected in May 2016 than they were in October 2015 (χ20.05, 316 = 7.769, P = 0.005). Our study documented the first presence of S. californiensis in Birch Creek as well as the level of infection in the Rainbow Trout population. Additional studies need to be conducted to determine the effect of this copepod on the Rainbow Trout in Birch Creek.