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1 Renewal-acute tests Acute exposure to dissolved selenium showed that razorback sucker were slightly less sensitive to the contaminant than fathead minnow, but that responses of the fishes were similar. The ratio of median lethal concentrations for razorback sucker (40.8 mg/L) and fathead minnow (33.3 mg/L) is about 1.2. Ratios between 0.5 and 1.5 are considered within the range of normal experimental variation (Mayer and Ellersieck 1986). Other investigators have compared sensitivity of razorback sucker and fathead minnow. 1 Finger et al. (1995) conducted studies that exposed both species to water from localities in the Green River basin receiving irrigation drainage. They concluded that sensitivity of razorback sucker was similar to that of fathead minnow. Dwyer et al. (1995) exposed razorback sucker and fathead minnow to a variety of contaminants and concluded that the two species were "generally similar". These consistent conclusions produced by different investigators using different sources of selenium, different contaminants, and different techniques represent strong evidence that razorback sucker and fathead minnow are approximately equal in sensitivity to dissolved ' selenium. Age of razorback sucker and fathead minnow used in renewal-acute tests were not the same (27 and 3 days after hatching, respectively). We intended to start acute toxicity tests coincident with the occasion when approximately 75% of razorback sucker began feeding. This event occurred approximately 10 days after hatching. It was chosen because it coincides with the time when larval razorback sucker probably first enter backwater or flooded bottomland habitats and exposure to relatively high selenium concentrations begins. Conflicts with simultaneously starting dietary and acute studies resulted in postponement of the latter. In general, the older fish 23