During long-term (1998-2000) studies on rodent parasite populations in Mazury lake district there were collected and analyzed data on co-occurrence of intestinal protozoa (Cryptosporidium parvum, Giardia spp.) and helminths. There were performed 178 autopsies of common vole Microtus arvalis, 85 autopsies of yellow-necked mouse Apodemus flavicollis and 386 autopsies of bank vole Clethrionomys glareolus. Positive effect of helminth infections was found in C. glareolus. Voles infected with nematode Heligmosomum mixtum showed higher prevalence of C. parvum and Giardia spp. than voles infected with Heligmosomoides glareoli. The host age took part in these interactions and positive effect of co-occurrence was mainly observed in voles older than 3 months. The other intrinsic (host sex) or extrinsic (season and year of study) factors influenced interactions between parasites. Presented results revealed that helminth infections may facilitate chronic infections of intestinal protozoa in rodent populations.