The use of pesticides to control pests and to increase food production is a normal process in this modern age. The objective of this study was to determine changes in different serum protein fractions caused by the action of malathion, a commonly used pesticide. We used Indian catfish, Heteropneustes fossilis. The fish were exposed to a sublethal dose of 4 mg/L of malathion for 24, 48, 72, and 96 hr. The LC50 value at 96 hr was found to be 11.676 mg/L. The results showed the formation of three low and four high mobility fractions and the disappearance of some protein fractions at different periods of exposure. The appearance of fractions A, B, and C (low-mobility proteins) may be due to altered immune responses caused by cellular damage. The appearance of new high-mobility fractions (D, E, F, and G) is possibly due to the breakdown of red blood cells and other cellular components. The findings of this study indicate that the high concentration of malathion (4 mg/L) induced more alterations in serum proteins compared with the low concentration (1.2 mg/L) used in our previous study.