In early rainbow trout development, a switch from high-affinity embryonic hemoglobin to lower-affinity adult hemoglobin occurs along with a turnover of round, embryonic erythrocytes to oval, adult erythrocytes. The objective of my thesis was to determine how the ontogeny of blood oxygen transport was affected by chronic hypoxia (30% of saturation) in rainbow trout. Hemoglobin-oxygen affinity, cooperativity, and the Bohr and Root effects were unaffected by hypoxia treatments, whereas hemoglobin content, erythrocyte counts, and hematocrit were significantly reduced. In hypoxia, larvae had higher concentrations of embryonic hemoglobin mRNA and embryonic erythrocytes than stage-matched normoxia-reared larvae. Overall, these results indicate that chronic hypoxia suppresses erythrocyte development prior to complete yolk absorption. Ultimately, this suggests that in early ontogeny rainbow trout conform to low oxygen conditions, rather than mounting the hypoxia response observed in oxygen-regulating adult trout.