Acute bioassays of pentachlorophenol were conducted with fathead minnows and goldfish to determine the effect of testing different sized fish of the same age or testing different aged fish. With different sized goldfish the rate of mortality during the test was similar but the threshold lc50 was higher for small fish. With different sized fathead minnows the rate of mortality during the test was significantly different but the threshold lc50 was the same for large and small fish. With different aged fathead minnows the rate of mortality was different for 14-week-old fish and the same for 11-, 7-, and 4-week-old fish but there was no difference in threshold lc50 due to age. Size selection of fatheads or goldfish within the ranges tested is unnecessary since differences are small. Since age of fatheads did not affect the 24-h, 96-h, or threshold lc50's, use of younger fish would allow smaller bioassay chambers or more fish per chamber.