In November 2000, an outbreak of Giardiasis intestinalis with aqueous transmission was recorded in one of the microdistricts of Perm. Thirty six children aged 1-7 years fell ill, 2 parasitic carriers were identified among the staff. The diagnosis was parasitologically verified in all children with clinical manifestations. Enzyme immunoassay was positive in 87.5% of the patients with the parasitologically verified diagnosis. The aqueous nature of the outbreak was confirmed by the detection of Lamblia cysts in the samples of water from the transfer network of the microdistrict in November 2000 and by the higher incidence of giardiasis among the microdistrict's residents since April 2000. Macmiror in age-specific doses was used to treat patients. The therapeutical efficiency of one course was 100%. If outbreaks of intestinal infections of unknown etiology occur, a parasitological survey should be made among patients and those who have contacted with them. Prolonged diarrhea was one of the major symptoms of giardiasis during an outbreak of this intestinal infection.