Technical Abstract: As United States and world seafood demand increases, aquaculture production becomes more important. Limited knowledge of molecular regulation of muscle growth and flesh quality hinders genetic improvement of these important traits in fish for aquaculture production efficiency and consumer satisfaction. Muscle growth is determined primarily by the balance between protein synthesis and degradation. When rates of protein synthesis are similar, protein degradation is critical in explaining differences in growth efficiency. In mammals muscle atrophy results from increased protein breakdown, and is associated with activation of the ubiquitin proteasome pathway, including induction of the muscle-specific ubiquitin protein ligase, MuRF-1. Animals lacking MuRF-1 are resistant to muscle atrophy. In fish, little is known about the role of the proteasome/MuRF pathway in muscle degradation. Our objectives were to: 1) clone and characterize MuRF genes in rainbow trout; and 2) determine expression of MuRF genes in association with starvation- and vitellogenesis-induced muscle atrophy in rainbow trout. We have identified full-length cDNA sequences for three MuRF genes (MuRF-1, MuRF-2, and MuRF-3). These genes encode proteins with typical MuRF structural domains, including a RING-finger, a B-box and a Leucine-rich coiled-coil domain. RT-PCR analysis showed that MuRF 1, 2, and 3 are predominantly expressed in muscle and heart tissues. Real time PCR analysis revealed that expression of all MuRF genes is up-regulated during starvation- and egg growth and development. These results suggest that MuRF genes have an important role in fish muscle protein degradation.