Technical Abstract: In the summer of 2000, isolates from natural infections of P. infestans (P.i.) were obtained from six wild species of Solanum planted ex-profeso under rainfed conditions in the Toluca Valley of México to determine possible host-pathogen specificity and for characterization regarding mating type (MT; A1, A2 ), sensitivity to metalaxyl, and the presence of allozymes of glucose-6-phosphate isomerase (Gpi) and peptidase (Pep). Solanum phureja and S. acaule presented diverse degrees of severity of the disease among individuals of each population. The rest of the species were uniformly resistant to the pathogen. Of 24 isolates from S. phureja, 13 multilocus genotypes were detected, the most frequent of which was the A2 100/100 Gpi and 100/100 Pep (22%) and an A1:A2 ratio of 0.6:1. Nine multilocus genotypes were found in the population of P.i. obtained from S. acaule. The most common one was also A2, 100/100 Gpi and 100/100 Pep (22% of such population too), with a MT proportion of 1:1. In S. bulbocastanum, no particular genotype was more common, and the only MT was A1, nor was there a more frequent genotype found in S. caripense, where the proportion of MT A1:A2 was 0.25:1. The only genotype recovered from S. muricatum was A1 86/11, 92/100 (Gpi, Pep), corresponding to US1. The two isolates from S. andreanum were A1, 86/100 Gpi and 100/100 Pep. Solanum phureja and S. acaule had the greatest frequency of isolates highly resistant to metalaxyl, and to a lesser degree of resistance those of S. caripense and S. muricatum. The isolates from S. bulbocastanum and S. andreanum, two species resistant to P.i., were A1 and susceptible to metalaxyl. All of the homothallic isolates were highly resistant to metalaxyl, and host-pathogen specificity was observed only in S. muricatum.