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Redband land Oncorhynchus mykiss gairdneri in the Columbia River Basin of western North America occupy desert and montane streams with variable habitat conditions. In general, desert streams are lower in gradient and elevation, contain less large substrate and more silt substrate, are less shaded by overhead vegetation, and have higher summer water temperature than montane streams. Consequently, we assessed whether the relationships between biotic and abiotic factors and the occurrence and abundance or redband trout in southwestern Idaho differed between desert and montane streams (<25 m mean width). Increased occurrence of redband trout in desert streams was most strongly related to increased stream shading and decreased amounts of silt substrate, followed Inn increased amounts of cobble/boulder substrate and absence of piscivorous fish (i.e., smallmouth bass Micropterus dolomieui and northern pikeninnow Ptychocheilus oregonensis). In montane streams, increased occurrence of redband trout was most strongly related to decreased site elevation and increases in cobble/boulder substrate, followed by decreases in stream gradient and width. Furthermore, occurrence of redband trout decreased in desert streams at mean stormier (June-August) water temperature (Temp(smr)) >16 degrees C, whereas for montane streams, occurrence increased at Temp(smr) >9 degrees C. Redband trout density in desert streams was most strongly related to higher stream order (re., headwater streams). increased stream shading, and increased amounts of cobble/boulder substrate. in montane streams, redband trout density was not well explained by any stream conditions, but stream shading had the strongest positive relationship with density. Redband trout density was negatively related to Temp(smr) in desert streams, nit he relationship was weak for montane streams. That environmental conditions related to the occurrence and abundance of redband trout differ between desert and montane streams is important fill fisheries managers Who manage these disparate populations occurring in such close proximity to each other