We studied the prevalence of Toxoplasma antibody over a 10-year period in a rural population of 326 people in Chorrera Province of Panama using the dye test. Fifty-five seroconversions were found in 108 people at risk, and 48 (87%) in children between 2 and 13 years with a mean incidence rate of 8.6% per year. Antibody prevalence rose from 25% at 5 years to 50% at 10 years of age, and increased gradually, reaching 90% by 60 years. Mean antibody levels after seroconversion were 1:6,000 in the dye test; they fell to 1:1,000 after 1 year, 1:800 after 2 years, 1:200 after 3 years, and 1:333 after 7–9 years. About 10% of antibody titers ranged between 1:4 and 1:32. Toxoplasma antibody prevalence was also studied in the metropolitan Panama City population using 590 sera collected in the fall of 1981. Age-specific incidence rates were similar in the urban and rural setting (correlation coefficient 0.71). The number of cats observed in the rural area and in the city and the degree of soil contact appeared compatible with a hypothesis of transmission by oocysts.