Small, medium, and large (mean total lengths of 30, 36, and 44 mm, respectively) hatchery-reared age-0 Colorado squawfishPtychocheilus luciuswere held at winter temperatures (3–5°C) in aquaria for 210 d to assess the role of size, condition factor, and lipid content on overwinter survival. Fish of each size-class were either fed dry pellets or starved. Fish in all size-classes fed at winter temperatures. The fish grew little, and condition declined as length of exposure to winter conditions increased. Condition declined more rapidly in starved than in fed fish of all size-classes. Lipid content of fish was inversely related to winter duration for all size-classes and feeding regimes. Percent survival at the end of 210 d was significantly lower in starved small (3.3%) and medium (6.7%) fish than in fed small (95.1%) and medium (98.4%) fish. Large fish showed 100% survival regardless of feeding regime. In years of large, prolonged runoff that delays the seasonal warming of the upper Colorado River, late spawning by Colorado squawfish leaves age-0 fish with only 2 months to grow and accumulate fat reserves before the onset of winter. These small age-0 fish may be unable to accumulate adequate lipid reserves to survive until the following spring.