The study was designed to investigate the relative efficacy of the guided inquiry and the expository teaching methods on the achievement in and attitude to biology of students of different levels of scientific literacy. Four research questions and four null hypotheses were posed and formulated respectively, to guide the work. It was hypothesized that effects due to teaching methods and their interactions with scientific literacy levels, were not significant (P less than 0.05), relative to students' mean achievement and attitudinal scores in biology. A pre-test, post-test, non-equivalent control group design was adopted for the study. One hundred and forty-seven Senior Secondary Two (SS11) biology students from eight intact classes, randomly selected from four secondary schools in Nsukka, Enugu State, Nigeria, constituted the sample. Three instruments namely: Scientific Literacy Test (SLT), Biology Achievement Test (BAT) and Attitude to Biology Scale (ABS), were used for data collection. The research questions were answered using mean and standard deviation scores, while the hypotheses were tested (P less than 0.05) using analysis of covariance (ANCOVA). The results showed that the guided inquiry method was significantly better than the expository method in enhancing cognitive achievement in biology for students of all levels of scientific literacy, especially the high ones. Students of different levels of scientific literacy showed positive attitude to biology, when the two methods were used. The interactive effects of teaching methods and scientific literacy levels, on both achievement in and attitude to biology, were not significant (P less than 0.05). The educational implications of the findings for biology teachers were highlighted.