Potato or late blight, caused by the patogen Phytophthora infestans is an important disease worldwide, and in Mexico too. In the present work monosporic progeny was obtained from single zoospores obtained from homothallic isolates and each of the single-spore isolates was met with reference isolates J104 (A1) and J204 (A2) to determine their mating type. We found that 56% of the progeny monosporic (zoospores) of homothallic isolates of Phytophthora infestans were homothallic. No progeny of A1A2 mating type was found whose characteristic is to produce oospores when faced with mating types A1 and A2, but is not able to produce oospores alone. Six homothallic isolates produced a 1:1 ratio in the mating types A1 and A2. The ability of the homothallic isolates of P. infestans to produce homothallic progeny through zoospores causes a quickly increase in the size of the population homothallic in the crop, allowing spreading to neighboring plots and thus increase the inoculum source of late blight. The number of oospores in crosses between homothallic isolates and heterothallic type A1 was 1.8 times greater than the number of oospores strictly produced in heterothallic crosses (A1 x A2). This indicates that populations of P. infestans probably produce a greater amount of homothallic oospores than a population where only heterothallic strains are found and increase the probability of sexual reproduction occurs in field and generate recombinant genotypes that are more pathogenic.