In order to estimate the structure of the loggerhead turtle assemblage in Okinawan waters, we measured the straight carapace length (SCL) and sexed 35 loggerhead turtles captured by set net between 2007 and 2011. Sexual maturity in females was determined on the basis of plasma triglyceride, cholesterol, total protein, and calcium concentrations. Of the 35 captured loggerhead turtles, 18 turtles were identified as males based on their elongated tails, while 16 turtles were identified as females in the process of vitellogenesis based on highly elevated blood plasma levels. Only one turtle with low plasma measurements was of indeterminate sex. We suggest that blood plasma measurements may serve as a useful indicator of female maturity. SCL means were 865 ± 48 mm (mean ± SD) for all turtles, 865±47 mm for males, and 871±52 mm for females; the turtle of unknown gender had an SCL of 791 mm. The loggerhead turtle assemblage in Okinawan waters was found to be larger in SCL and higher in the frequency of mature individuals than those in conspecific assemblages in other areas of Japanese waters so far studied. We suggest that the loggerhead turtles migrating to Okinawan waters are primarily in active reproductive states in preparation for mating and nesting.