The spore replication rate of the parasitic microsporidian, Nosema bombycis, was investigated in the pupal and adult stages of two popular tropical silkworm races of India, Pure Mysore (multivoltine race) and NB18 (bivoltine race). The rate was assessed in terms of spore yield per unit weight of tissue. In pupae, spore replication in the bivoltine race was higher than that in the multivoltine race. Spore replication in both races followed a logistic pattern. Spore production increased substantially with age of moths in both races. The cephalothoracic region of each race had the highest spore concentration, especially around the wing and wing muscles.