Water soluble organic compounds (OCs), including naphthenic acids (NAs), are potentially toxic constituents of oil sands process-affected water (OSPW) that is generated during extraction of bitumen from Alberta oil sands. Ozonation can decrease concentrations of OCs in OSPW. However, effects of ozonated-OSPW on multicellular organisms are unknown. A 10-day and a chronic exposure of Chironomus dilutus to OSPW were conducted to assess effects on survival, growth, development, and behavior. Two separate batches of OSPW were treated with 30 or 80 mg ozone (O(3))/L. Wet body masses of larvae exposed to OSPW were 64 to 77% less than their respective controls (p < 0.001). However, both levels of ozonation significantly attenuated effects of OSPW on growth. Similarly, chronic exposure to untreated OSPW resulted in significantly less pupation than in the controls, with 31% and 71% less pupation of larvae exposed to the two batches of OSPW (p < 0.05). Emergence was significantly less for larvae exposed to OSPW, with 13% and 8% of larvae emerging, compared to 81% in controls (p < 0.0001). Both levels of ozonation of OSPW attenuated effects on emergence. These results suggest that OCs degraded by ozonation causes toxicity of OSPW toward C. dilutus, and that ozonation attenuates toxicity of OSPW.