Prevalence of antibodies against Giardia duodenalis was determined by enzyme-linked immunosorbent assay in serum samples from a national serologic survey of Mexico that included all geographic areas and socioeconomic and demographic data for each person sampled. The country was divided into four regions on the basis of development (high, medium high, medium low, and low). Of 3,461 serum samples tested, 1,914 (55.3%) were positive for IgG antibodies against Giardia duodenalis. Seropositivity was age-specific; the probability of seropositivity increased 4.9-fold (95% confidence interval [CI] = 3.16-7.64) in adolescents 10-19 years of age, 8.0-fold (95% CI = 5.19-12.53) in young adults 20-39 years of age, and 12.6-fold (95% CI = 7.93-20.28) in adults more than 40 years of age. Giardia duodenalis seropositivity was associated with male sex (odds ratio = 1.40, 95% CI = 1.22-1.61). No association was found between seropositivity and socioeconomic variables or regional development status.