The house sparrow Passer domesticus L. has become established and spread in almost all oceanic islands and continents, including northern Europe, during the last two centuries. This bird is a convenient subject to study the fate of parasites during the spread of their hosts to new geographical areas and ecosystems. We surveyed the house sparrow from St Petersburg Zoological Garden (36 birds) and from the village Rybachy on the Curonian Spit in the Baltic Sea (91 birds) during the breeding season in June and July 1996 and 2003, respectively. The birds were investigated by microscopic examination of blood films. The overall prevalence of blood parasites was 34%. Species of Plasmodium, Leucocytozoon, Isospora, Hepatozoon, and microfilariae were recorded. There was no significant difference in the prevalence of all infections between the two study sites. Haemoproteus spp. were not seen at either site. Species of Plasmodium were seen only in adult birds. Isospora adiei was significantly more prevalent in young than in adult birds. The results of this study are in accord with the literature data on the general pattern of distribution of blood parasites in the house sparrow in other parts of its extended range. It is probable that the spread of the house sparrow to new ecosystems contributes to the spread of I. adiei and Plasmodium spp., but not of Haemoproteus spp. The scarcity of Haemoproteus spp. infections in the house sparrow in northern Europe, North and South America, sub-Saharan Africa, Australia, and oceanic islands show that this bird may be resistant to the majority of haemoproteids, which are common in the local avifauna. The application of molecular techniques is needed for understanding the phylogenetic relationships of blood parasites of the house sparrow and other birds.