Screening of a large number of different diploid Solanum accessions with endosperm balance number (EBN) 1 revealed segregation for strong resistance and sensitivity to Phytophthora infestans in accessions of Solanum mochiquense. Genetic analysis showed that resistance in S. mochiquense accession CGN18263 resides at the distal end of the long arm of chromosome IX, is linked to restriction fragment length polymorphism marker TG328 and is in the neighbourhood of the quantitative trait locus (QTL) Ph-3 conferring resistance to P. infestans in tomato. This is the first genetic study of S. mochiquense, a wild diploid species originating from fog oases in the Peruvian coastal desert.