The growth and reproductive biology of the invasive goldfi sh Carassius auratus auratus were studied in Lake Trasimeno, central Italy. The results of the research revealed that the population is made up of eight age-classes. The sex ratio proved to be extremely unbalanced (1 : 19 in favour of females). Growth can be deemed rapid, the von Bertalanffy growth in length function being TL = 43.019{1- e(-0.272(t+0.162) )} for the total sample and Ф’ = 2.702. No sexual dimorphism in growth was observed. Back-calculation analysis suggested the existence of an inverse Lee phenomenon among 1 year-old specimens. The reproductive period covers a broad time-span, from March to June. In females, sexual maturity is reached after the second winter of life (2+ age-class); however, a small percentage (7.55%) of females is able to reproduce at the age of 1 year. Most of the males attained sexual maturity in the fi rst year (60.61%). The reproductive investment of the females is high; the relationship between SL and the number of eggs was Ne = 0.0041 SL4.368. Fecundity varied from 286 to 219104 eggs, with an average relative fecundity of 103 ± 5 eggs g-1; the mean diameter of the eggs was 1.27 ± 0.01 mm. The reproductive investment of the females was not homogeneous across the ageclasses; in addition to absolute fecundity, relative fecundity and egg diameter were seen to increase with the size of the specimens.