The proportion ofShigellainfections that occur asymptomatically in young children has not been established. A community-based cohort study of 367 infants was followed prospectively by weekly home visits from January, 1990, through December, 1991. Stool samples were collected weekly and when diarrhea occurred and were tested forShigellaand other enteropathogens. There were 2925 child months of observation and 65 episodes ofShigellainfection. There were 3.1 episodes/100 child months during the warm season (May through September) and 0.97 episode/100 child months during the cold season.Shigellainfections were rare during the first 6 months of life but increased with age (P< 0.0001). Overall 55% of detected infections were asymptomatic. The proportion of infections that were asymptomatic increased as age increased (P< 0.01). Symptom status was not significantly associated withShigellaspecies or season. All isolates from symptomatic and asymptomatic children had the 120− to 140− megadalton virulence plasmid. We conclude that infections with virulent strains ofShigellaare commonly asymptomatic in Mexican children during the first 2 years of life.