This investigation ascertained the effect of modular instruction on the performance in Biology of freshmen fishery students at the Carlos Hilado Memorial State College (CHMSC) - College of Fisheries during the second semester of the school year 2009-2010. Experimental research using the pre - test, post - test control group design was employed to achieve the objectives of the study. Participants were grouped into the experimental and the control groups. The experimental group was exposed to modular instruction while the control group was exposed to lecture-discussion approach. The pre-test performances of both experimental and control groups were fair. The post-test performance of the experimental group taught by modular instruction was superior while that of the control group taught by lecture-discussion was very good. A significant difference was found between the pre - test of the modular and lecture-discussion groups in favor of the latter group. A significant difference was found between the post - test results of the control group in favor of the experimental group. Significant differences were found between the pre-tests and post -tests of the experimental and control groups hence, modular instruction was better than the lecture -discussion approach in effecting students’ performance in Biology.