Topmouth gudgeon (Pseudorasbora parva) is one of the most invasive organisms in recent times. It can spread very quickly due to its attributes, which predetermine its success in novel environments. One source of these attributes appears to be phenotypic plasticity. The reproductive parameters of a long-term established population from a stable habitat exposed to a strong disturbance were the subject of the present study. The results supported the prediction that such a strong disturbance will increase the absolute number of oocytes and decrease their size significantly. Of course, changes in life-history can be observed in any species of fish, regardless of whether the disturbance occurs in a native or an introduced habitat. However, the hypothesis of alternative ontogenies and invasive potential presumes that the wider the range between the most generalized and the most specialized phenotypes that a species can generate, the wider is its invasive potential. The observed shift in reproductive traits demonstrates the high phenotypic plasticity of topmouth gudgeon.