| Summary: | The study of the biological diversity of the Arctic zone yields intriguing results. Initial research on the lakes of the Yamal Peninsula resulted in the identification of <i>Chironomus laetus</i> and the hybrid <i>Ch. laetus</i> × <i>Ch</i>. sp. Le1. To avoid misidentification, we used morphological, cytogenetic, and molecular genetic approaches. By cytogenetics, in <i>Ch</i>. sp. Le1, seven banding sequences were found: Le1A1, Le1B1, Le1C1, Le1D1, Le1E1, Le1F1, and Le1G1. The karyotype of <i>Ch. laetus</i> was mapped for the first time; it is the first species with the arm combinations AE BC DF G. We propose the name of a new cytocomplex—“laetus”. DNA-barcoding of the <i>COI</i> gene was carried out for <i>Ch. laetus</i> and <i>Ch. laetus</i> × <i>Ch.</i> sp. Le1 for the first time. The estimated genetic distance between the sequences of <i>Ch. laetus</i> and <i>Ch. riihimakiensis</i> is 2.3–2.5%. The high similarity in morphology, banding sequences, and the possibility of hybridization indicate a close relationship between <i>Ch. laetus</i> and <i>Ch</i>. sp. Le1, which is assumed to be the northern variant of <i>Ch. riihimakiensis</i>. Molecular genetic data suggests the presence of a subgroup with <i>Ch. laetus</i>.
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