The Mexican potato (Solanum tuberosum L.) crop is affected by several diseases, including purple top (PPT). PPT is caused by phytoplasma and is transmitted by insects in the Cicadellidae family. In severe infections, this pathogen can cause yield losses of nearly 100%. The objective of this research was to identify and classify (at a molecular level) the causal agents of PPT. Potato plants were sampled in the state of Mexico (at Tenango del Valle, Zinacantepec, Atlacomulco and Donato Guerra) in the spring and summer of 2006. Approximately 40% of the plants tested positive for phytoplasma. The presence of plant DNA was determined by amplifying an endogenous gene with the 16s-1/16s-2 primers. To detect phytoplasma, the P1/P7 universal primers were used for PCR, the R16F2n/R16R2 primers were used for nested PCR, and the AluI, HpaII, and KpnI enzymes were used to perform RFLPs. The results indicated that the phytoplasma associated with the PPT disease belongs to the aster yellows (16Sr1) group and the I-B subgroup. The same phytoplasma was found in all three regions of study. When the sequence of the amplified fragment was compared with the registered Gene Bank (NCBI) sequences (using the BLAST package) a homology of 99% was found with the aster yellows (16Srl) group, "Candidatus Phytoplasma asteris".