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Reproductive biology of males in the Guam population of the Brown Treesnake, Boiga irregularis, was investigated through monthly examinations of the urogenital system organs and plasma testosterone levels. All males examined during the 12 consecutive months of the study were spermatogenic and had sperm in the ductus epididymis and ductus deferens. No evidence of testicular recrudescence or regression was observed. Testis mass did not vary among months. Epithelial height of the kidney sexual segment was the only feature examined that varied significantly among months, with lowest heights observed in May through July. Despite this variation, the sexual segment in all males was hypertrophied and contained secretory granules. Plasma testosterone levels did not vary significantly among months, were relatively low compared to those of most other snake species, and were extremely variable among individuals. There were individuals with near-zero levels in most months. Overall, the reproductive biology of males on Guam is aseasonal. In light of this finding, the claim of seasonality in the putative source population is reassessed. Testosterone levels were independent of coelomic fat body mass, which was extremely variable among males and, in many cases, quite low. Observations on morphological features suggest that individual reproductive capacity increases disproportionately with increasing body size. The possibility of facultative aseasonal/seasonal reproduction in the species is discussed. The observed capacity for continuous spermatogenesis and its relative independence from body condition may facilitate the invasive capabilities of this ecologically damaging species.