Increased incidences of mortality and adverse effects have been described for wildlife exposed to oil-sands-process-affected waters (OSPW). Naphthenic acids (NA) were identified as a primary toxic component of OSPW, yet little information exists regarding NA-induced toxicity in aquatic vertebrates. Amphibian larvae may be particularly susceptible to exposure to OSPW in groundwater surrounding oil sands regions, and increased frequency of mortality and adverse developmental effects were noted in exposed tadpoles. Despite this, there are no published studies investigating the effects of NA exposure on developing tadpoles. LC50 values of 4.76 mg/L NA were found for tadpoles at an early developmental stage (Gosner stage 28), and even greater toxicity with more developed tadpoles at 96 h, with an LC50 value of 3.04 mg/L in Gosner stage 36 tadpoles. These values are well below NA concentrations found in OSPW tailing ponds and similar to levels identified in groundwater in the Athabasca Oil Sands region.