Blood samples of more than 1,100 passerineform birds of 40 species were investigated for the occurrence of microfilariae. In the year 2005, 3 out of 677 birds of 31 species (prevalence 0.4%) were infected with microfilariae during the post-nesting period. During the pre-nesting period in the year 2007, 11 out of 438 birds of 31 species were infected with microfilariae (prevalence 2.5%). Both the pre-nesting and post-nesting examinations were conducted at the same location in the northeastern part of the Czech Republic. The microfilariae of the Eufilaria delicata and Ornithofilaria mavis species were found in Turdus merula, Turdus philomelos, and Erithacus rubecula (Passeriformes, Turdidae). Single individual of Poecile montanus (Passeriformes, Paridae) was infected with undetermined microfilariae. The morphometric variability of microfilariae found in T. philomelos, E. rubecula, and Poecile montanus were recorded. Infections caused by microfilariae E. delicata were more frequent than infections caused by O. mavis. Seven adult nematodes E. delicata were found in a subcutaneous cyst on the heel joint in one T. philomelos, which is the first record of adult E. delicata nematodes in birds in the Czech Republic.