In an outbreak of giardiasis at two day-care nurseries in metropolitan Toranto Giardia lamblia appeared to be transmitted person to person. No common source of infection could be found. The proportions of children infected in the two nurseries were 39% and 17%; infection was spread to 7% and 23% of their household contacts. Of the infected children and household contacts 26% and 30% respectively of those for whom detailed information could be obtained were symptomatic. Canadian children were more likely to be infected and symptomatic than were immigrant children attending the nurseries. The most susceptible ages for infection were 1 to 3 years. The results of this study suggest that all children in day-care nurseries who are infected with G. lamblia should be treated, regardless of whether they are symptomatic.