Group A Kunitz-type protease inhibitors (KPI-A) are involved in protecting potato plants from microorganisms and pests. While the nucleotide sequence is known for many KPI-A genes of various potato cultivars (Solanum tuberosum subsp. tuberosum) and a few genes of tomato (Solanum lycopersicum), there are no data on their allelic diversity in other species of the genus Solanum. KPI-A fragments were cloned, amplified, sequenced, and analyzed from plants of the subgenera Potatoe sect. Petota (five genes from S. tuberosum ssp. andigenum and two genes from S. stoloniferum) and Solanum (five genes from S. nugrum), and their consensus sequences were established. An identity of 97-100% was observed among these sequences and the KPI-A sequences of the sections Petota (cultivated potato Solanum tuberosum ssp. tuberosum) and Etuberosum (S. palustre) The interspecific variation of KPI-A did not exceed its intraspecific variation for all but one species (S. lycopersicum). The distribution of highly variable and conserved sequences in the mature protein-coding region was the same in all of the above species. The same primers failed to amplify the homologous genes from Solanum dulcamara, S. lycopersicum, and Mandragora officinarum. Phylogenetic analysis of the KPI-A sequences showed that S. lycopersicum clustered separately from all of the other species examined, that S.nigrum clustered together with species of the sections Etuberosum and Petota, and that these species produced no species-specific clusters. Although S. nigrum is resistant to all known races of the oomycete Phytophthora infestans, which causes one of the most economically important diseases of Solanaceae, the amino acid sequences encoded by S. nigrum KPI-A differed slightly, if at all, from their counterparts of cultivated potato, which is susceptible to P. infestans infection.