Instream toxicity tests using the larval fathead minnow Pimephales promelas and the cladoceran Ceriodaphnia reticulata were conducted on Prickly Pear Creek, Montana waters to study toxicity persistence in a stream. The toxicity source was Spring Creek, a tributary of Prickly Pear Creek. Gold mining tailings and settling ponds in the Spring Creek drainage release zinc, copper and cadmium to Prickly Pear Creek via Spring Creek. Stream survey characterization of flow regimes, water quality, and biotic conditions was accomplished in conjunction with toxicity testing. The study objectives were to: (1) develop a data base for validation of a toxicity persistence model; (2) assess the applicability of data from the Prickly Pear Creek study relative to model assumptions; and (3) assess field techniques for acquiring model input data. Toxicity to the test organisms was primarily due to zinc and copper in Spring Creek waters.