At low flows, ambient toxicity (measured as a reduction in Ceriodaphnia survival) was observed in two tributaries and in the main channel of Turkey Creek. Toxicity during periods of no observable stream flow was absent in Turkey Creek upstream of these sources and reduced downstream of them and a wastewater treatment plant. During stormwater runoff, ambient toxicity increased throughout Turkey Creek, but especially at the downstream sites. Toxicity was not markedly altered at the two tributary sites where there was significant toxicity under runoff and non-runoff conditions. The toxicity from the tributaries was attributed to the presence of zinc, nickel, or cadmium as implied by a follow-up analysis of a toxicity identification evaluation (TIE), known land use, and ambient water quality data. Overall, the aquatic toxicity observed in Turkey Creek is likely due to sources in the streambed, unconsolidated urban runoff, mine tailings, or synergistic effects.