Factorial studies were conducted to determine the effects of variable concentrations of calcium, magnesium, sodium and potassium on the uptake of dissolved strontium-85 by small bluegills, Lepomis macrochirus. The distribution of strontium-85 introduced to a pond containing macrofauna indigenous to the Midwest has also been investigated. Only calcium of the four cations tested was shown to have a significant effect within the experimental range on the uptake of strontium-85 by small bluegills. Data from the pond uptake study indicate that after the 80-day experimental period only 25 per cent of the strontium-85 remained in the dissolved phase. The total calculated activity in the experimental fauna never exceeded 0.5 per cent of the amount added. The remaining radioactivity was associated with the substrate. These data also indicate that strontium-85 accumulation is related to age of the test organisms.