The study examined the interaction of logical reasoning ability (cognitive development) and socio-economic status on achievement in genetics amongst secondary school students in Nigeria. Factorial Analysis of variance design with one dependent variable and two independent variables at two levels together with the t-test was used in the analysis of the result. Two hundred and forty senior secondary three (S.S.S III) students randomly selected from six secondary schools in Ethiope East and Okpe Local Government Areas of Delta State, Nigeria, were used as subjects for the study. These were randomly assigned to four groups of sixty students per group. Four research instruments were developed and validated for the study. The result showed that the variables of cognitive development and socio-economic status have positive significant relationship with achievement in Biology. More over, it was also found that cognitive development and socio-economic status are two separate interactive independent variables in science education. (Contains 3 tables.)