The effect of salinity and pH on the immune response of the white shrimp Litopenaeus vannamei was studied over a period of 12–15 days. The results indicate that salinities between 5‰ to 30‰ and pH of approximately 7.0–9.5 affect hemocyte count, phenoloxidase activity, bacteriolytic activity and antibacterial activity. Phenoloxidase activity peaked at the 12th hour, whereas bacteriolytic activity and antibacterial activity were lowest. By the 6th day of exposure, every parameter returned to the values observed in controls except hemocyte count, which remained low with a decrease in salinity. From the 6th to the 15th day of the salinity test, the hemocyte count at every salinity were constant but significantly lower than control; significant changes in phenoloxidase activity, bacteriolytic activity and antibacterial activity were observed over the same period. During the first 3 days exposure to variable pH, hemocyte count and antibacterial activity decreased gradually as time elapsed at pH 7.0, 7.5, 9.0 and 9.5. Phenoloxidase activity peaked at the 12th hour, but the bacteriolytic activity fell. During the third to the 12^th day of the pH test, every immune parameter was stable. The hemocyte count, bacteriolytic activity and antibacterial activity decreased, whereas the phenoloxidase activity increased in response to a change in pH.