This paper presents the original data on studies of populations of guppies on the territory of Ukraine on the example of those of them surviving for many years in the drainage system of Kyiv. For 10 years, wild populations of guppies and their morphological features were studied in the warm water flows of the Bortnychi aeration station in Kyiv (2011–2020). During this period, the original “key” was developed to describe the morphology of their coloration peculiarities, which includes: total length, the number of pattern and coloration elements (4 types, 9 elements), pigmented area (light — orange, dark, pigmentation index), the shape and pigmentation of the tail (6 types) and its asymmetry At present, in the countries of Eastern Europe, the species is not adapted to any waters in the wild due to low winter temperatures, but these fish have become well established in sewage and other warm water bodies in urbanized areas (cities, towns, factories, etc.). We described the places where this invasive species had been found for many years, highlighting its key features. Thanks to GIS modelling, it was revealed that the existence of wild populations of guppies in Ukraine and Latvia is possible only in warm waters (currently mainly sewage systems of big cities) within anthropogenic territories. Such a key and study of relatively isolated populations of invasive species will provide a deeper understanding of micro-evolution of their morphological features (coloration) in isolation, help to track distribution of invasive species in a changing climate and provide material for comparison with possible similar invasions in areas heavily affected by military actions.