Inorganic monophosphates supplementation effects in high plant ingredient-based diets of juvenile Oncorhynchus mykiss were evaluated by feeding four experimental diets supplemented with 11.4, 10.0, 11.5 and 10.0 g/kg of monocalcium phosphate (MCP), monoammonium phosphate (MAP), monopotassium phosphate (MKP) and monosodium phosphate (MSP), respectively. After 60 days, MAP and MKP supplemented groups showed improved growth and feed intake (p > .05). Apparent digestibility coefficient of dry matter (ADC DM) was significantly higher in MSP diet followed by MKP, MCP; MAP diet showed significantly lower ADC DM. MAP and MSP diets showed significantly higher mineral digestibility. The postprandial plasma P levels were significantly influenced by the dietary treatments and the period after a single meal. MAP, MKP and MSP groups showed significantly higher baseline P concentration than MCP. The other macro-minerals (Ca, Mg, K and Na) were significantly influenced by the postprandial time. MSP and MKP diets showed significantly higher lysozyme activity. Total immunoglobulin was significantly higher in MSP group. MKP diet showed significantly higher expression of cd4-2β genes compared with MCP. Significantly higher lysozyme gene expression observed in MSP diet followed by MKP; MAP and MCP groups showed significantly lower value. Finally, MAP, MKP and MSP significantly improved nutrient digestibility, mineral bioavailability and immune functions of rainbow trout compared with MCP in high plant ingredient-based diets. K E Y W O R D S digestibility, high plant ingredients, immunity, monophosphates, postprandial absorption, rainbow trout | 2179 HOSSAIN et Al.