This article presents the results of comparative analysis and the morphological as well as morphometric characteristics of two species, Gambusia affinis, and Gambusia holbrooki of the family Poeciliidae commonly found on the plains of the republic's water bodies; in the shallow coastal regions of rivers and lakes. During the research, 44 fish specimens were used, with body lengths ranging from 21 to 55 mm. Their dominance over other species was monitored. The morphometric analysis of the samples provided data on the plastic and meristic characteristics of the studied species. Differences between species were mainly observed among male individuals in terms of meristematic characteristics. At the same time, the number of fin rays in Gambusia affinis was detected in the following order: on the dorsal II 5, anal – 10, on the ventral – I 5, on the thoracic – III 8, and on the caudal – 23-24, and the number of scales on the lateral line was 32-33. The number of fin rays in Gambusia holbrooki was determined in the following order: on the dorsal – II 6 (5), anal – 10, on the ventral – I 5, on the chest – III 8, and on the caudal – 22-23, and the number of scales on the lateral line was 31-32. The coefficient of variation for plastic characteristics in the male Gambusia affinis varied from 3.9% to 38.1% (4.0% on average), while that of the male Gambusia holbrooki, the recorded variation was from 2.2% to 23.7% (on average 5.2%).