The two coefficients of the von Bertalanffy growth model [maximal standard length (L(S infinity)) and initial growth rate (a)] were determined in eastern mosquitofish Gambusia holbrooki in the most northern area where it is established (Rennes, Brittany, France). The estimated mean L(S infinity) was 57.7 and 25.0 mm in females and males, respectively. The value of a was identical for both males and females before the males diverged from the females by slowing down their growth, probably because of the onset of puberty. The sexual competition reported in males would be responsible for higher interindividual variability of L(S infinity) in males than in females. a was positively correlated with water temperature and could be modulated by fish density and length at birth. Moreover, interindividual variability of a was influenced in a complex way by interactions between water temperature, length at birth and fish density. When compared to the data from studies in G. holbrooki inhabiting their native range or the southern introduced range, the present results suggest that the thermal tolerance domain for growth was altered and that males exhibited a shorter length at sexual maturity and a shorter L(S infinity) in the population established in Rennes.