The western mosquitofish(Gambusia affinis) is one of the most invasive fish species worldwide, with well-documented ecological impacts on ecosystem function and several native taxa. This species was introduced to Shanghai, China in 1924 to control mosquito larvae, and has since invaded aquatic habitats across many parts of China, including the middle Yangtze River. However, the population parameters of western mosquitofish, particularly in China, are relatively unknown. In this context, we determined its population parameters in Wuhan, located in a typical middle reach of the Yangtze River. We surveyed western mosquitofish from two typical water bodies in Wuhan from April 2012 to March 2013. We collected 1489 fish, which included 1069 females, 406 males, and 14 juveniles. Based on length frequency data collected from the survey, growth, mortality, and recruitment pattern of the mosquitofish were estimated using FiSAT II software developed by Food and Agriculture Organization of the United Nations(FAO). The results can be summarized as follows:total length of G. affinis population ranged from 8.30 to 44.86 mm, 16-26 mm being the most common, and accounting for 69.17% of the whole population. The overall sex ratio of females to males was 2.63:1, significantly different from 1:1, and the ratio varied depending on the month. The ages of the fish captured were determined by scale analysis. Two age groups(0⁺, 1⁺) were identified in females, but only one age group(0⁺) was identified in males. The relationship between total length(L, mm) and body weight(W, g) can be expressed as W=4.686×10^-6 L^3.299(R²=0.953) for females and W=1.008×10^-5 L^2.989(R²=0.903) for males, and the slopes(b value) of the total length-body weight relationship indicated positive allometric growth for females and isometric growth for males. The estimated parameters for the von Bertalanffy growth equation were L_∞(asymptotic length)=45.20 mm, k(growth coefficient)=0.52 a^-1, t₀=-0.47 a for females and L_∞=33.60 mm, k=0.53 a^-1, t₀=-0.50 a for males. Our results showed that asymptotic length was larger in females than in males, and this may be attributed to their relatively longer longevity. The instantaneous rates of natural mortality(M) calculated for females and males were 1.64 a^-1 and 1.81 a^-1, respectively. The length-converted catch curve method gave a total annual instantaneous mortality(Z) of 2.29 a^-1 for females and 2.37 a^-1 for males. These results indicated that natural mortality(M) and total mortality(Z) were quite close. There are two possible reasons for this:(1) this small fish species is not fished because this species has no economic value, and(2) the water bodies we surveyed were strictly protected by local government, and no fishing was allowed, so unintentional fishing for mosquitofish was also not possible. The recruitment pattern was continuous, displaying a single major peak event per year. These results demonstrate that western mosquitofish is a small size fish with a short life span, rapid growth rate, high natural mortality, and a long recruitment period. These characteristics indicate that this species is a very typical r-strategy species. We suggest local governments should prevent its introduction, and detect and eliminate this species as early as possible.