The brown bullhead is a fish native to North America that became an invasive species after being introduced into the waters of Europe and other regions. Studies on its sexual cycle and fecundity were conducted on a population from an oxbow lake of the central Vistula River in Poland. The fish ranged in age from 1 + to 9+. The average body length (SL) was 14.4 ± 3.4 cm. Individuals as young as 1 + were already mature. Females lay eggs multiple times from mid-April to mid-June, with absolute fecundity averaging 3227 oocytes and relative fecundity at 46 oocytes g−1. The highest mean GSI of 1.9 during spawning was recorded in June. The reproductive tract of males takes the shape of lobes and consists of a paired cranial region formed by testes and a caudal region of undefined function. In males, semi- cystic spermatogenesis occurs, with secondary spermatocytes leaving cysts. Males overwinter with tubules filled with spermatids and initiate spermatozoa formation. The highest average GSI of 0.49 was recorded in early April. The brown bullhead observed in the new habitat was characterized by multiple egg laying, earlier maturation of individuals, and spawning in the earlier part of the calendar year, i.e. mid-April to mid-June, compared to its native habitat