Short-term lethality tests were conducted with five (waste oil, No. 1 fuel oil, No. 2 fuel oil, mixed blend sweet crude oil, Lloydminister crude oil) oils and four freshwater species. The oils were tested as floating layers, emulsions, and as the water-soluble fraction of 10% oil-water mixtures, in static and flow-through tests. The organisms tested were the American flagfish, Jordanella floridae, the fathead minnow, Pimephales promelas, larvae of the wood frog, Rana sylvatica, and larvae of the spotted salamander, Ambystoma maculatum. LC50 values were quite variable depending on a number of influencing factors, including the species tested, the type of oil, differences between batches of the same oil, the form of the oil when added to the test system, the type of test, duration of exposure, and the oil-water contact time.