To further assess the hypothesis that differential hemoglobin isomorph formation may be involved in the respiratory response of rainbow trout (Salmo gairdneri), yearling animals were first rendered acutely anemic by immersion in phenylhydrazine HCl. Erythropoiesis was then induced by gradual warming under normoxic and hypoxic conditions. While no significant differences in hemoglobin content were observed, trout exposed to hypoxia displayed transient, but significant differences in hematocrit. Significant variations in both the occurrence and abundance of specific isomorphs were also observed.