With nest incubation temperature determining sex in marine turtle hatchlings, sex ratios among populations and intermediate aggregations are likely to exhibit more geographic and temporal variability than for vertebrates with genotypic sex determination. In this study, we examined the sex ratio of an immature Hawksbill Seaturtle (Eretmochelys imbricata) aggregation at Mona Island, Puerto Rico, using serum testosterone level criteria that were validated through laparoscopy in a subset of turtles. Measured serum testosterone for female turtles ranged between 0.32 and 17 pg/ml and for males between 18.2 and 262 pg/ml. Of 120 turtles sampled, 53 individuals were classified as females and 66 as males, with one individual undetermined. The resulting F:M ratio of 0.80:1 is not significantly different from 1:1. This result contrasts with the highly female-biased sex ratios reported from surrounding Caribbean immature hawksbill aggregations, suggesting that the Mona near shore habitat recruits male turtles from a source uncommon to other aggregations.