Direct underwater observation was used to quantify numbers and locations of eight species of fish in 100-m sections along a 1,200-m reach of Sagehen Creek, California, at biweekly intervals between 5 July and 22 September 1983. Most of the eight species exhibited significant short-term changes in abundance and reached peak abundance at different times. Total fish abundance was highest in mid-July; the Margalefindex ofdiversity was highest in mid-August. Lahontan redsideRichardsonius egregius, Tahoe suckerCatostomus tahoensis, and mountain suckerC. platyrhynchusappeared to migrate from a downstream reservoir into the stream during the spawning season. In addition, fish were not distributed evenly in the study reach. All species tended to prefer pool habitat and pools were not uniformly distributed. Relative fish composition determined at long-term permanent sections by electrofishing was not significantly associated with most underwater counts made at different times or stations. This study illustrates mobility of an entire fish assemblage and indicates how long-term studies of fish populations may be greatly influenced by the timing and placement of samples.