Studies of the median toxicity thresholds for 10 species of fish in brine wastes from a single oil well showed a range of the thresholds from 4.3 to 11.2 percent of the original brine by volume. A similar study on 10 kinds of invertebrates revealed a range of thresholds from 1.8 to 8.7 percent. Thresholds were expressed as percentage volume of the original waste since the brine contained large amounts of calcium, magnesium, potassium, and sodium as well as chloride ions. However, computations of hypothetical combinations of chlorides in the brine indicated sodium chloride to be present in such proportions as to be most toxic.