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Ten isolates of entomopathogenic nematode (EPN) Heterorhabditis indica were evaluated for their differential infectivity against two major cotton insect pests viz., Helicoverpa armigera and Spodoptera litura. The results of bioassay revealed that the third instar larvae of H. armigera were relatively more susceptible to the EPN isolates compared to the larvae of S. litura. All the ten EPN isolates could reproduce successfully on H. armigera larvae however, the isolates viz., CICR-Su and CICR-SUB exhibited relatively higher virulence. On the other hand, on S. litura larvae only one isolate- CICR-BBFNS2 was able to produce infective juveniles and the rest failed to do so. Analysis of haemolymph composition of EPN infected H. armigera and S. litura larvae indicated the phase change of bacterial symbionts as a primary reason for the failure of EPN to reproduce and multiply in S. litura larvae. Higher percentage of primary phase colonies of bacterial symbionts were recorded in H. armigera larvae which resulted in successful progeny production of EPN while secondary phase colonies were predominant in the larvae of S. litura which resulted in failure of EPN isolates to reproduce. Phylogenetic tree made using internal transcribed spacer (ITS) region of ribosomal DNA of nematodes and 16sRNA of bacterial symbionts revealed the distinctness and uniqueness of the EPN isolate CICR-BBFNS2 which was able to multiply on the larvae of both H. armigera and S. litura from rest of the nine isolates which could multiply only in H. armigera. The results indicate that isolate CICR-BBF-NS2 has potential for deployment in IPM programme for management of H. armigera and S. litura. [ABSTRACT FROM AUTHOR]