The oocysts of Cryptosporidium parvum isolated from Microtus arvalis, Clethrionomys glareolus and calves were used in laboratory experiments carried out on 35 M. arvalis. Oocysts were detected in 43% of 14 voles infected with C. parvum derived from calves and in 54% of 13 voles infected with C. parvum isolates from C. glareolus. All voles inoculated with oocysts from M. arvalis (MA-voles) developed infection. There were observed differences in courses of infection between the three groups of voles with MA-voles demonstrating the most typical curve with an acute phase of infection in the first 4 weeks and a 2-weeklong phase of self-curing. The infections with 'heterologous' C. parvum isolates (C. glareolus and calf isolates) transformed into chronic infections with lower oocyst productions. A strong host age effect was observed on prevalence of C. parvum among voles of different ages. Eight-month-old voles did not become infected and in older animals the infections were milder. The successful cross-transmission of two C. parvum isolates to M. arvalis suggests that this species should be considered as reservoir host for this important parasite of humans and animals.