In 1984, the Rio Blanco Oil Shale Company received permission from the US Bureau of Land Management/Oil Shale Projects Office to proceed with retort abandonment activities at its Tract C-a modified in situ retort site. One of the first abandonment activities undertaken was to flood the retort with groundwater to dissolve soluble contaminants associated with the retorting operation. Saline water was then pumped from the retort into evaporation ponds during two pumpdown operations in May of 1985 and June of 1986. The principal objective of the pumpdown operations was to remove contaminated groundwater from the retort area and to prevent the migration of contaminants beyond the retort. A toxicological evaluation of groundwaters collected from within the retort and outside the retort is currently in progress. Acute and chronic toxicity tests have been performed using the freshwater invertebrate Ceriodaphnia affinis/dubia with groundwater samples collected before and after the first pumpdown of the retort. The objectives of these tests have been to evaluate the success of the pumpdown operation, to assess the effect of the pumping operations on groundwater quality both within and outside the retort, and to evaluate the toxicity of groundwater within the retort relative to local groundwater that has not been affected by the retorting operation. This report presents the results of toxicity tests performed before and after the first pumpdown operation. Additional toxicity tests are planned for samples collected after the second pumpdown operation. 15 refs., 2 figs., 9 tabs.