The winged mapleleaf mussel [Quadrula fragosa (Conrad)] is a Federal endangered species. Controlled propagation to aid in recovering this species has been delayed because host fishes for its parasitic glochidia (larvae) are unknown. This study identified blue catfish [Ictaluris furcatus (Lesueur)] and confirmed channel catfish [Ictaluris punctatus (Rafinesque)] as suitable hosts. The time required for glochidia to metamorphose and for peak juvenile excystment to begin was water temperature dependent and ranged from 28 to 37 d in a constant thermal regime (19 C); totaled 70 d in a varied thermal regime (12-19 C); and ranged 260 to 262 d in simulated natural thermal regimes (0-21 C). We developed a quantitative model that describes the thermal-temporal relation and used it to empirically estimate the species-specific low-temperature threshold for development of glochidia into juveniles on channel catfish (9.26 C) and the cumulative temperature units of development required to achieve peak excystment of juveniles from blue catfish (383 C???d) and channel catfish (395 C???d). Long-term tests simulated the development of glochidia into juveniles in natural thermal regimes and consistently affirmed the validity of these estimates, as well as provided evidence for a thermal cue (17-20 C) that presumably is needed to trigger peak juvenile excystment. These findings substantiate our model and provide an approach that could be used to determine corresponding thermal criteria for early life development of other mussel species. These data can be used to improve juvenile mussel production in propagation programs designed to help recover imperiled species and may also be useful in detecting temporal climatic changes within a watershed.