In 1987 the University of Guadalajara performed a seroepidemiological survey on the prevalence of Chagas’ disease in the 124 counties of the State of Jalisco, Mexico, arriving at a rate of 21.6 per 100 inhabitants. From December 1993 to June 1994, we studied 2238 individuals from 32 rural counties in this State. Of these, we found 276 positives (12.33%) and 1962 negatives (87.66%). Nevertheless, the series of serological differences found are very striking, since out of the 655 individuals that were seropositive in 1987, we noted that 276 individuals remained positive, while 50 individuals (7.63%) became negative. There were no flaws in the laboratory techniques. We believe that either the immune response of Mexicans is different or that the virulence of the Mexican strains of Trypanosoma cruzi may be not as great as that in the South America countries.