A mosaic karyotype 78,XX/77,X was found in two infertile purebred dogs. Cytogenetic investigations were carried out on a large number of metaphase spreads (220 and 473) with the use of conventional Giemsa staining, C-banding, and chromosome painting approaches. The frequencies of the monosomic spreads (77,X) were low: 5.3% and 5.6%. We recommend that at least 90 metaphase spreads be analyzed to allow possible detection of low-level XX/X mosaicism, and we discuss the need for cytogenetic evaluation of dogs with reproductive disorders.