Abstract : The study sought to examine the effect of the implementation of the constructivist learning model on teaching the two units" Organizing Life's Diversity, Bacteria and Viruses" to first year secondary school students compared with the traditional method. In order to achieve the objectives of the study, a semi-experimental design was used. The study was applied on a sample of (68) first year secondary school students in the Kingdom of Bahrain. The sample has been divided into two equal groups: One of the two groups, the experimental group, studied "Organizing Life's Diversity, Bacteria and Viruses" in line with the constructivist learning model. The other, the control group, studied the same unit in accordance with the traditional method. An achievement test on the two units" Organizing Life's Diversity, Bacteria and Viruses" was applied on the sample of the study before and after the treatment. Data were analyzed using the statistical package for the social sciences (SPSS). The study has come to the following conclusions: 1- There is statistically significant differences at (0.05) level between the two mean scores of the experimental group students who studied using the constructivist model and the control group students who studied using the traditional method at the level of the overall grade of the achievement test. 2- There is significant statistical differences at (0.05) level between the mean scores of first year secondary school students at the level of the overall grade of the achievement test. It is attributable to the achievement level of students ( high, medium, low). 3- There is no significant statistical differences at (0.05) level in the achievement of first year secondary school students attributed to the interaction between the teaching methods and the students achievement level. The study put forward a number of recommendations: 1- The adoption of the constructivist learning model by teachers, supervisors and officials in the field of science education. 2- Training teachers to implement the constructivist model in teachings science subjects. 3- The inclusion of constructivist methods models in textbooks in general and biology textbooks in particular. The study also suggested a number of related studies.