A grey-hooded parakeet (Psilopsiagon aymara) and two budgerigars (Melopsittacus undulatus) from different owners presented with decreased activity, vomitus, and diarrhea. A microscopic examination of feces showed trophozoites of the protozoan flagellate Giardia. A commercial immunochromatographic dipstick test for Giardia sp. antigens confirmed the infection. These findings were assured by PCR of the small subunit ribosomal RNA (SSU rRNA) gene and coproantigen ELISA. Sequencing of PCR products of the SSU rRNA (292 bp) and β-giardin genes (511 bp) identified Giardia psittaci as the species involved. Therefore, our results show that a GSA 65–based coproantigen ELISA, which was established for diagnosis of Giardia duodenalis is applicable for the detection of G. psittaci. A treatment with ronidazole was started. Additionally, fecal examination and dissection of the dead birds revealed coinfection with the fungal pathogen Macrorhabdus ornithogaster. One budgerigar survived and repeatedly tested negative after treatment with ronidazole. The described cases indicate that a single infection with G. psittaci has a good prognosis, whereas the prognosis is poor when coinfections occur, especially with M. ornithogaster.