Some biological properties of eperythrozoon, such as the influence of low temperature on their viability, their infectivity in different kinds of animals and their changes in amount of organisms in vivo were investigated in the present study. The experimental results showed that the viability of eperythrozoon could be maintained at -20 ℃ for 835 days and at 4 ℃ for 205 days. Small rats could be infected by Eperythrozoon suis, while chickens were not infected. The basic regulation on the changes in amount of organisms in animals was expressed by the fact that the percentages of erythrocytes being infected increased rapidly up to 96-98% 2-4 hours after infection, then it dropped down and followed by dropping and re-rising, attaining its climax up to 8 hours after infection. Finally, it reflected a fluctuating cycle of 14-16 hours. This results showed that eperythrozoons can maintain their viability at -20℃ for a long time and they possess relative species specificity to host with a reproduction cycle of 14-16 hours in animal body.