This study was part of a larger project designed to test for the effect of lime (calcium hydroxide) on the persistence of trifuralin (in the herbicide Treflan) in the water of farm dugouts. We report here the results of an investigation into the effect of lime on the benthic invertebrate population in a Treflan treated dugout. A dugout in the Peace River region of Alberta was divided into havles by a seacurtain (polyethylene). Both sides were treated with Treflan (9.27x10^-4 l/m³ ) and one side with lime (100g/m³ ). A six-inch Eckmann Dredge was used to take twenty benthic samples per side prior to treatment and sixteen samples per side three and eight weeks after treatment. Family Chironomidae and Subfamily Chaoborinae were the most prominent invertebrates present in the benthic samples. Prior to treatment, neither Chironomidae nor Chaoborinae showed a statistical difference in population density between the two sides of the dugout. However, the first post-treatment samples (three weeks) showed a significant reduction in the density of Chironomidae on the Treflan-lime side as compared to the Treflan only side (p<0.001). There was no significant change in Chironomidae's wet weights throughout the study. Chaoborinae showed a similar, but not statistically significant (p<0.1), trend. It is concluded that the addition of lime to a Treflan containing dugout may result in a reduction in its benthic invertebrate population.