The study was undertaken to characterise the C. parvum isolates originatingfrom naturally infected woodland and field rodents: Clethrionomys glareolus (CG), Apodemus flavicollis(AF) and Microtus arvalis (MA). We found that the measurements of oocyst dimensions and oocyst morphologydid not allow distinction between the parasite isolates from the 3 rodent species. The mean dimensionswere: for CG 4.67 x 4.21, for AF 4.65 x 4.14 and for MA 4.66 x 4.16. These 3 groups of isolates haveproduced significantly different pictures of infection in C57BL/6 mice. The overall mean oocysts outputwas: in CG-mice 41,739, in AF-mice 18,000, in MA-mice 10,384 oocysts/1 g of faeces. From these data wesuggest that rodent isolates of C. parvum could represent new subgroups in so-called "mouse" C. parvumstrain. The successful cross-transmission from wild hosts to laboratory rodents and the close similarityof COWP sequence between our isolates and "mouse" genotype and between "mouse" and zoonotic genotypeof C. parvum (genotype C) inform us that all these isolates should be treated as potentially hazardousfor human health.