Gambusia holbrooki (Girard, 1859) is worldwide spread invasive fish species and is found in the wetland, the Lake Van basin. This study was carried out to determine the biological characteristics of G. holbrooki. A total of 544 specimens were caught by electrofishing between August 2019 and July 2020. The maximum total length were 5.52 cm for females and 3.10 cm for males. The length-weight relationships were calculated as W = 0.014L(2.609) (R-2 = 0.808) for males (negative allometric growth) and W = 0.009L(3.110) (R-2 = 0.964) for females (positive allometric growth). The Fulton condition factor was determined as 1.00 +/- 0.01 (0.47-1.87). The female: male ratio was calculated as 1.75 : 1.00. The mean number of eggs for females was determined as 42.2 +/- 2.3 (15-94). Egg diameter ranged from 0.92 to 2.82 (mean 1.72 +/- 0.40) mm. The mean gonopodium length for males was 6.60 +/- 0.20 (5.90-7.20) mm. In general, due to high reproductive capacity and competitiveness, G. holbrooki are a serious threat to native freshwater fauna of the Lake Van Basin.