The study was carried out to investigate the effect of inquiry-based teaching and the lecture method on students’ achievement in Biology in senior secondary schools in the Delta Central Senatorial District. Four (4) research questions were raised, and four (4) hypotheses were formulated and tested at a 0.05 alpha level of significance. The study adopted pre-test and post-test non-equivalent planned variation as a quasi-experimental design. The study sample consisted of 325 biology students, selected using stratified and simple random sampling procedures from schools. The Biology Achievement Test (BAT) was the instrument used in this study and validated by three Experts. Reliability coefficients of 0.76 was established for BAT. The data collected were analyzed using descriptive statistics and independent sample t-test and ANCOVA. The findings of the study are: (i) there is a significant difference in mean achievement and interest scores between students taught with inquiry-based teaching and lecture method, (ii) there is no significant difference between mean scores of male and female students exposed to inquiry-based teaching method on achievement amongst others. It was recommended that inquiry-based instruction be encouraged among Biology teachers to raise students' dwindling interest in the subject and improve students’ achievement.