This paper presents the ocurrence of homothallic isolates of Phytophthora infestans within potato crops treated with metalaxyl. Isolates of P. infestans were obtained from potato crops within the municipality of Zinacantepec, State of Mexico (Mexico). Each isolate was confronted with the reference isolates J104 (A1) and J204 (A2) and cultivated in the same medium just to check the homotallism. The measurement of the metalaxyl resistance of the isolates studied was determined under the methodology proposed by the FRAC (Fungicide Resistance Action Committee), where the isolates that showed more than 40% growth relative to the control in the presence of 5.0 mg (active ingredient) of metalaxyl / ml were considered resistant. Results show that two out of the five isolates collected in 2011 were homothallic. Regarding the fifteen isolates collected in 2012, four were homothallic. Such homothallic isolates were resistant to metalaxyl. Keeping in mind that the studied isolates were sampled in potato crops treated with metalaxyl and that showed resistance to this fungicide, the results suggest a possible relationship with and use of metalaxyl and the homotallism. However, it may not be the only reason because heterothallic isolates from the same origin are also resistant to metalaxyl.