The study was undertaken to characterise the C. parvum isolates originating from 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 morphology did not allow distinction between the parasite isolates from the 3 rodent species. The mean dimensions were: 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 have produced significantly different pictures of infection in C57BL/6 mice. The overall mean oocysts output was: in CG-mice 41,739, in AF-mice 18,000, in MA-mice 10,384 oocysts/1 g of faeces. From these data we suggest that rodent isolates of C. parvum could represent new subgroups in so-called "mouse" C. parvum strain. The successful cross-transmission from wild hosts to laboratory rodents and the close similarity of COWP sequence between our isolates and "mouse" genotype and between "mouse" and zoonotic genotype of C. parvum (genotype C) inform us that all these isolates should be treated as potentially hazardous for human health.