This study investigated the Effects of Learning Together and Jigsaw II Learning Strategies on Upper Basic II Science Students’ Achievement and Retention in Nasarawa State, Nigeria. Two research questions and two hypotheses guided the study. This study adopted a quasi-experimental research design. It is a pre-test, post-test and post-post-test, non-equivalent, non-randomized control design. Simple random and purposive sampling technique were employed to draw 88 Upper Basic II science students in Nasarawa State West Senatorial District as sample for the study. Basic Science Students’ Achievement Test (BASSAT) consisting of 40-multiple choice items was uses as instrument for data collection, the reliability of the instrument was determined using Kuder-Richardson formula 21 (K-R21), The coefficient of internal consistency for BASSAT was 0.82. The data collected from the administration of instrument was analyzed using the following statistical techniques. Descriptive statistics, which involve the computation of the pretest, post-test and the post-post-tests mean scores, standard deviation were used to answer research questions and Analysis of Covariance (ANCOVA) was used to test hypotheses at 0.05 level of significance. The findings of this study reveals significant difference between the achievement of students in Learning Together and Jigsaw II learning strategies, in favour of the Learning Together group. Also reveal significant difference between the retention of students in Learning Together and Jigsaw II learning strategies, in favour of the Jigsaw II group. Based on the findings of this study it was recommended that; Basic Science teachers should be encouraged to develop and adopt Learning Together and Jigsaw II Learning Strategies so as to improve and promote social interaction active learning, discovery learning, motivation, learning by doing and learning by experience among students which will lead to enhanced overall achievement and retention..