Early developmental stages of fish mostly depend on innate immune factors for their protection. Augmenting these factors by application of different immunostimulatory substances may be beneficial for rearing and survival of the early life stages of fish. Bath administration of stimulants leads to a uniform exposure of fish independent of feed intake and reduces the individual handling. The present study demonstrates the immunostimulatory effect of β-glucan (bath exposure) in rainbow trout fry at different dosages and exposure time. Rainbow trout fry (avg. wt. 770 mg; 87 days post hatch) were exposed to three different concentrations of β-glucan (10, 100 and 1000 μg mL−1) by bath exposure for 1 and 24 h. Expression of immune related genes from pooled internal organ samples of individual fish were analysed using a real time qPCR assay. Expression of complement factors (C3 and factor B) and acute phase proteins (hepcidin, precerebellin and transferrin) was significantly up-regulated after 24 h bath stimulation with β-glucan (100 μg mL−1). These innate immune factors may play a vital role in clearance of pathogens. The expression of most of genes showed both a dose- and time-dependent response. A medium dose (100 μg mL−1) induced a significant increase in expression of complement factors and acute phase proteins mainly at 24 h exposure, whereas the highest dose of β-glucan (1000 μg mL−1) down-regulated the expression of most of the studied genes. The result from the present study indicates that β-glucan bath exposure could be applied for enhancing the innate immune factors even in fry.