This study examined the effects of various types of wastewater produced in oil-sand-refining on the survival, hematology, gill morphology, and swimming of caged fathead minnows (Pimephales promelas Rafinesque, 1820). At the reference site, all fish survived a 28-day exposure with unchanged hematocrit, leucocrit, and gill histology. In con- trast, all fish did not survive a 28-day period in any of the wastewaters tested and, in some cases, they had all died within 96 h. In addition, the hematology or gill morphology of fish that had survived shorter exposure durations was found to be significantly altered; the changes included a significant decrease in lymphocytes and significant gill cellular hyperplasia and hypertrophy. The present data suggest that water remediation will be needed before the process wastewater from oil-sand-refining can support fish populations.