Urbanization typically increases water temperatures in streams and rivers via decreased canopy cover and warming of stormwater runoff. Increased temperature is hypothesized to be a major contributor to the decline of many species at risk, including the redside dace Clinostomus elongatus, but data are lacking. Redside dace populations are at risk through much of their range (listed endangered in Canada), but healthy populations remain in Ohio. We are studying these robust Ohio populations to determine acute thermal tolerance (CTmax) of juvenile and adult fish throughout an annual cycle to inform Canadian recovery efforts. During the summer (stream temperature ~22°C), CTmax was significantly higher in juveniles (35.5°C) than adults (34.5°C). Within juveniles, CTmax was positively correlated with body mass, but within adults this relationship was negative. Body condition (Fulton’s K) was not related to CTmax. These results indicate that, during peak summer temperatures, the largest adult redside dace are the most thermally sensitive. Temperatures in historic redside dace habitat in southern Ontario can approach the CTmax values we measured when pavement- warmed stormwater is discharged directly to streams. Thus, our data suggest that restoration strategies that lower water temperatures in these streams should be emphasized.