Abstract: This paper describes the cryopreservation of microfilariae of Brugia malayi for 48 tours by means of the cryogenic technique of Ham's two-step cooling schedule. The experiments suggested that the viability of the microfilariae, as illustrated by motility (%), normal appearance (%), and mean activity index (maximum = 4) was lower in the frozen groups than in the unfrozen controls, whether or not newborn calf serum had been added. The viability of the microfilariae treated by cryogenic technique with newborn calf serum was higher than that without. We observed that the viability of microfilariae was greatly decreased with the increase of DMSO concentration within 5-30%. The results showed that the optimal cryopreservation solution was pH 7.0-7.2 Tyrode's solution containing 5% DMSO as cryoprotectant and 20% newborn calf serum as well as 200 units/ml of penicillin and streptomycin each and maximum viability occurred after cooling at the rate of 1℃ per minute to -17℃, followed by rapid plunge into liquid nitrogen.