ABSTRACT  This study was to investigate the effect of mixed ability grouping on senior secondary students’ academic achievement in Economics in Rivers State. Four research questions and four null hypotheses guided the study. The study adopted a quasi-experimental design precisely the non-equivalent control group design. The sample ofthe study consisted of 79 senior secondary II students (41 male and 38 female students). The study adopted purposive sampling to ensure that schools with similar characteristics were used in this study. The instrument for data collection was a multiple-choice question titled; “Economics achievement Test” (EAT). The instrument was subjected to content validation using table ofspecification while the KuderRichardson (20) Formula was used to determine the internal consistency of the items that made up the instrument which yielded a correlation coefficient (r) of 0.81 which indicated that the instrument was reliable. The research questions were answered using mean and standard deviation while analysis of covariance (ANCOVA) was used to test the null hypotheses at 0.05 level of significance. Results showed that students who studied in a mixed-ability group performed better than those students who studied in an ability group. The study revealed that mixed-ability, grouping improved the achievement scores ofstudents and also ability grouping had more increasing effect on male students than their female counterparts among others. Based on the findings, the researcher recommended among others that school administrators should adopt grouping of their students based on mixed ability since the finding has proved it to be more effective in students’ achievements than the ability grouped classes due to the fact that it provide cross fertilization of ideas among the categories of achievers.