Cooperative learning (CL) is a learning style that refers to small, heterogeneous groups of students working together to achieve a common goal. This involves student-student interaction within small groups in a way that each group member’s success is dependent on the group’s success. Self-regulated learning (SRL) style believe on what ‘knowing’ is and how one ‘comes to know.’ SRL recognizes that individuals may to some extent control their own learning through contexts, relationships, and situations. The purpose of this study was to determine how these two learning styles impact on students’ achievement in Biology. The influence of sex and ability (high and low) were also investigated when comparing CL and SRL styles.. The study was carried out in third term of 2013/2014 academic session in Bayelsa State, Nigeria on senior secondary 2 students aged 14-17 years. The biology ability test question (BATQ) which was adopted from standardised West African examination council (WAEC) past examination question papers on the topics in the syllabus that were covered within the period of the research, was used for data collection. Significant higher achievement test scores were observed for students of varying abilities in cooperative learners in comparison with self-regulated learners; No significant difference in test scores achievement was observed between the male and female students. Moreover, no significant interaction effect was observed between sex and ability, sex and method, ability and method and among method, sex and ability on achievement. The implication of this in teaching/learning of Biology is that teachers should model their instructions to enforce student – student interaction.