Gastrointestinal nematodes (GINs) cause significant morbidity and financial losses to sheep enterprises. An appealing control strategy is to reduce GIN parasitism by identifying and breeding sheep with a superior immune response to GINs. However, most screening methods for GIN immunity were developed in warm temperate climates with year-round GIN exposure. Regions with long, cold winters that interrupt GIN exposure, such as Ontario, Canada, are under- represented in studies of GIN immunity. Moreover, limitations of existing GIN speciation methods preclude characterization of species-specific immunity. Therefore, the objectives of this thesis were to: 1) validate GIN speciation using deep amplicon sequencing; 2) assess effects of GIN parasitism on productivity of sheep grazing in Ontario; 3) evaluate salivary antibody to carbohydrate larval antigen (sCarLA) as a method of assessing GIN immunity in Ontario sheep; and 4) generate preliminary information on the relationship between acute stress responsiveness, cell- and antibody-mediated immune responses, and GIN parasitism. Deep amplicon sequencing of first-stage larvae (L1s) generated comparable results to traditional morphologic speciation of third-stage larvae (L3s). Furthermore, significantly higher proportions of eggs developed to L1s than to L3s. Productivity and sCarLA were monitored in 140 replacement ewe lambs for two years in a commercial flock in central Ontario. Subclinical GIN infection had minimal effect on productivity of those ewe lambs. Even so, most of the animals developed detectable sCarLA by the end of their first grazing season; these levels waned over winter but reached higher levels in the second grazing season, suggestive of an anamnestic response. Additionally, sCarLA was always negatively associated with GIN fecal egg counts, and levels in individual animals were generally consistent over time. No significant differences in GIN parasitism were identified between lambs with high and medium acute stress responsiveness, or between lambs with immune responses biased towards antibody- or cell-mediated immunity. The data presented in this thesis will be used to direct wider-scale validation of sCarLA in selection of Ontario sheep with optimal GIN immunity.