Variation in esterases activity in different silkworm races and hybrids were investigated to determine the usefulness of this marker for the assessment of genetic variation among them. A study on the haemolymph esterases pattern of different silkworm populations from different origins (two races and five hybrids) kept in Egypt was undertaken for enhancing the breeding program in the Sericulture Research Department (SRD). The larval haemolymph showed esterolytic activity capable of hydrolysing α and β naphthyl acetate substrates. A total of 14 α and 10 β – esterase and 21 α and 24 β-esterase bands were detected during the fourth and the fifth larval instars, respectively. By evaluating the degree of similarity among the tested populations, it was found that similarity of α esterase between Race 2, Hybrids 2 and 3 during the fourth larval instar was 50%. The similarity percentage was 40% between Race 2 and Hybrid 5 during the fifth larval instar. For β esterase, the similarity was 66% between Race 2 and both Hybrid 1 and Hybrid 4 during the fourth larval instar and was 66% between Race 2 and Hybrid 5 during the fifth larval instar. It may be recommended to dispense Race 2 from the breeding program as it shows the highest degree of similarity with the other tested populations. At the physiological level, esterases appeared to be a suitable marker to estimate the genetic variation among Bombyx mori races and hybrids.