Disease tolerance occurs when pathogens and parasites have minimal e ffect on host fitness. The degree of tolerance is typically quantified by the relationship between pathogen load and host survival or reproduction. Dermo disease, caused by the protozoan parasite Perkinsus marinus, negatively impacts survival in both wild and cultured eastern oyster (Crassostrea virginica) populations . Several studies have demonstrated breeding oysters that survive in Dermo-endemic environments leads to higher population-level survival in subs equent generations; however, it is un clear whether the increased survival is due to evolved resistance or tolerance to the disease. Here we describe a disease challenge experiment designed to 1) test for evolved tolerance among selectively-bred eastern oyster families and 2) characterize phenotypic and transcriptomic responses to the parasite. E astern oyster families exhibiting a range of field survival phenotypes were obtained from a well-established breeding program and each family was divided into four groups . The groups were exposed to distinct doses of P. marinus (0, 106, 107, and 108 parasite spores per gram wet weight) via injection in the adductor muscle and monitored for survival every day for 50 days. Individuals ( n = 3 - 6) from each family/ dose combination were censored seven days post exposure and mantle tissues were collected to confirm parasite load (via qPCR) and generate global gene expression profiles (via RNAseq) . mRNA samples from two families at each extreme of the dose-response tolerance spectrum were ultimately selected for short-read, paired-end Illumina sequencing. On average, 93M reads per sample were generated and 93% of paired reads from each sample mapped to the eastern oyster genome. Counts of reads mapped to each feature (transcript) of the nuclear genome were used to quantify gene expression. Principal component analysis of gene expression profiles showed significant clustering of samples by treatment (control vs. injected) along PC1 and by phenotype (tolerant vs. susceptible) along PC 4. To assess the effects of family, phenotype, and dose on the transcriptomic response to Dermo, comparisons between control and treated sample expression were made at multiple levels (e.g. each family and dose separately , families grouped by phenotype at each dose, and all doses combined for each family). Depending on the comparison, hundreds to thousands of differentially expressed transcripts were detected. Analysis of overlapping differentially expressed transcripts found a higher percentage of overlapping transcripts among doses within each family than among families within a single dose, suggesting that the host genotype/phenotype may be more important than parasite load in driving the transcriptomic response to Dermo . More detailed analyses of differentially expressed transcripts, functional annotation and enrichment are underway. Taken together, the results of this study should provide valuable insights to this host/parasite interaction and the mechanisms underlying Dermo tolerance in the eastern oyster.