Introduction and objective: Toxoplasma gondii is a widespread protozoan parasite that infects human and animals. Tissue cyst of T. gondii is an important source of human and other hosts infection including cats as definitive host. This study was carried out to determine growth of T. gondii tissue cyst in laboratory syurian mouse.

Materials and methods: Fifteen mice in three groups were infected with T. gondii by intraperitoneally inoculation. The mice of each group killed after two, three and four months. Ten smears of brains were prepared from each group and examined microscopically for Toxoplasma tissue cyst. The diameter of 17 cysts from each group was determined and recorded and the mean size in each group calculated.

Results: The mean diameters of tissue cysts were 49.4×44.8µm after two months and 64.5×55.7µ after three months of inoculation. After four months the mean diameter of tissue cyst reached to 60.6×53.48µ. The volume of tissue cysts grew within two and three months after inoculation but after four months the growth leveled off. 

Conclusion: The present study showed that the mean diameter of tissue cyst grew during the first three months and after four months became stable and some may get compacted and lesser.