We investigated giardiosis of dogs and cats with a view to cysts in fecal samples and antibodies in the blood serum. Cysts were found in 25 fecal samples (5.5%) out of 458. Cysts were excreted frequently in puppies and adolescent dogs. In 15 cases giardiosis was diagnosed together with canine distemper or parvovirosis as an opportunic infection. Indirect immunofluorescence was used for detection of specific antibodies. We tested sera of 588 dogs and specific antibodies were detected in 156 cases (36.5%) in titres 10-160. In basic dilution 1:10 antibodies were detected in 61% positive samples. Titres 20-160 were found in 39% of the positive sera. In cats cysts were found only in 1 fecal sample out of 135 due to the difficulties in obtaining fecal samples for repeated examinations. Specific antibodies were detected in 107 cats (56.9%) out of 188 at titres 10-320. In basic dilution 1-10 antibodies were detected only in 30% of the positive sera but on the other hand in 70% we detected higher titres, i.e. 20-320. We did not consider titre 10 as significantly positive in both dogs and cats because cross-parasitic infections frequently occur. The number of serological positive dogs and cats grew with their age because they acquired giardiosis in the course of their life mostly as asymptomatic infection. The detection of specific antibodies cannot be used for the diagnosis because their level in blood serum persists for months. We must tread every dog and cat excreting cysts in fecal samples to stop transmission to other animals and humans-zoonosis.