M.S. University of Hawaii at Manoa 2013. ; Includes bibliographical references. ; Demand for tilapia is growing fast in the United States, positioning tilapia as the second best-selling fish in retail stores behind salmon. In Hawaii, Oreochromis aureus and Oreochromis hornorum tilapias are easily available, but currently, little is known about the somatic growth characteristics of the two tilapia species. Therefore, we designed a study: 1) to examine the growth characteristic of two tilapia species (O. aurea and O. hornorum), 2) to examine the biochemical characteristics related to somatic growth in two tilapia species at different growth stages. Tilapia were raised in outdoor green water tanks in Magoon Facility, UH Manoa. Body weight and length were monitored during the 12 month grow-out period. Skeletal muscle samples were collected at 4, 7, 10 and 12 months from each species, and were frozen immediately in liquid nitrogen for later analysis. O.Hornorum grew faster thanO.Aureus during 12 months growth period. A wide variation in body weight was observed in both species with the coefficient of variation being 19.8% for O. aureus and 24.9% for O. hornorum, respectively. O. hornorum fillet harvested at 12 month had significantly higher protein (20.0% vs. 17.9%) and fat percentage (0.93% vs. 0.71%), but lower percentage of water (78.0% vs. 80.4%) than O. aureus fillet harvested at 12 month without any significant difference in ash percentage. SDS-PAGE analysis showed that little degradation of muscle proteins happened during cold storage for 7 days. Muscle RNA and DNA concentrations of O. hornorum and O. aureus declined as they grow in size. Muscle DNA concentration of O. aureus was higher than that of O. hornorum regardless of growth stages. However, O. hornorum had higher muscle RNA concentration and RNA: DNA ratio than those of O. aureus regardless of growth stages. Since RNA concentration and RNA: DNA ratio are closely related to protein synthetic capacity, it is speculated that differences in protein ...