Innate immune factors play a crucial role in survival of young fish especially during early stages of life where adaptive immunity is not fully developed. In the present study, we investigated the immune response of rainbow trout larvae (Onchorhynchus mykiss) at an early stage of development. We exposed 17 days post hatch (dph) larvae (avg. wt. 70 mg) to the bacterial pathogen, Yersinia ruckeri at the concentration of 1.0 X 108 cfu/ml for 4 h. Samples were taken at 1, 4, 12, 24, 48, 72 and 96 h post infection for qPCR and immunohistochemical studies. In the same experimental trial, another group of naive larvae were bath exposed for 1 h with different concentrations of ß-glucan from Euglena gracilis. Two days post infection, larvae showed typical signs of Yersinia infection with haemorrhages in mouth region and inflammation on the dorsal side of the body. However, no mortality was observed during the experimental period. Expression pattern of different cytokines, complement factors, acute phase proteins, antimicrobial peptides, lysozyme, immunoglobulin genes were analysed by using qPCR. It is interesting to note that neither infected nor stimulated larvae showed any significant up/down regulation of studied genes compared to the control group. Immunohistochemical studies showed a high expression of IgT molecules in gills and CD8 positive cells in thymus. The present study hints towards the presence of a special protection mechanism in these young larvae; probably without regulating the expression of immune factors at the transcriptional level. It may be speculated that at this stage of life, larvae may combat invading pathogens by using armour consisting of different immune factors without regulating their expression.