Stream temperature is one of the most important physical parameters affecting growth and development of amphibians, especially cold-adapted stream dwelling amphibians such as tailed frogs. We characterized the annual thermal regime of streams at the northern range of the Rocky Mountain tailed frog (Ascaphus montanus) and associated it with tailed frog distribution and biomass. We monitored stream temperatures between August 2008 and August 2009 at 41 sites ranging from small headwater streams to larger valley bottom streams. At a subset of these sites (n = 27), we collected tailed frog abundance and biomass estimates. Two temperature metrics, maximum weekly average temperature (MWAT) and accumulated degree days > 5 °C (DD₅), a measure of physiological time, were related to tailed frog occurrence and abundance. Larval occurrence was found to be limited when DD₅ was ≤ 160 °C and MWAT ≤ 8 °C. No breeding streams were too warm to support tailed frogs. Frogs and tadpoles had similar realized niche centres for both MWAT (about 10.8 °C ) and DD₅ (about 355 °C ). However, frogs had greater realized niche widths than tadpoles. The temperature metrics were only weakly related to catchment-scale variables (area, mean elevation, ruggedness, aspect, site elevation), such that catchment scale variables cannot be used to make reliable predictions of thermal habitat suitability. This study provides a greater understanding of the thermal conditions required for tailed frog occurrence and may help managers predict potential range shifts resulting from climate change or land use and more effectively conserve tailed frogs into the future.