The purpose of this quantitative comparative study was to determine if and to what extent there was a statistically significant difference in the achievement of mathematics and English language arts (ELA) among male and female students who attended single-sex schools from those who attended mixed-sex schools in Caribbean countries. The theoretical foundation was the dimensions of gender achievement which focused on the Male-Female math and ELA gap, diving into which is more stereotypical and gender-favorable. The research questions asked if there were statistically significant differences in the achievement of male students and female students who attended all-male schools and all-female schools from male students and female students who attended mixed-sex schools, respectively, on the 2018 CSEC mathematics and ELA exam. In this study, data were analyzed using the Mann-Whitney U Test, a nonparametric test. There were statistically significant differences in the achievement of males attending all-male schools and males attending mixed-sex schools. There also were significant differences in the achievement of females attending all-female schools and females attending mixed-sex schools. Males and females in single sex performed better on the test in both subject areas. The study showed U(N[subscript male-only] = 83, N[subscript mixed-sex] = 83,) = 6014.000, z= -4.00, p<0.001 on the mathematics examination and U(N[subscript male-only] = 83, N[subscript mixed-sex] = 83,) = 6208.500, z= -3.01, p<0.001 on the ELA examination for male students who attended all-male and mixed-sex schools. The study showed U(N[subscript male-only] = 83, N[subscript mixed-sex] = 83,) = 5366.000, z= -5.21, p<0.001 on the mathematics examination and U(N[subscript male-only] = 83, N[subscript mixed-sex] = 83,) = 5820.000, z= .523, p<0.001 on the ELA examination for female students who attend all-female and mixed-sex schools. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]