The larva and karyotype of a Xenochironomus sp. in North America is described from two localities in Kansas. Its karyotype and larval morphology are differentiated from Xenochironomus xenolabis (Kieffer, 1916) and Xenochironomus sp. occurring in Russia. Differentiating characteristics of the larvae are lack of small teeth adjacent to the central tooth of mentum, and flagellum of antenna extending to the beginning of fourth segment of the antenna. Differentiating characteristics of the karyotype are 2n = 8, with polytene chromosomes combined in chromocentres of different degrees of stability, species-specific chromosome band sequences, and location of active regions. The species is very polymorphous, with every chromosome arm having complex heterozygous inversions occurring in high frequency. Chromosome rearrangements found in the species are discussed and the relation with other Xenochironomus species is proposed.