Abstarct: This ex post facto causal-comparative study compared vocabulary acquisition and oral proficiency between Grade 4 pupils taught by subject-specialist English teachers and those taught by generalist classroom teachers in Baghdad primary schools. Two research questions guided the study: (a) What difference exists in vocabulary acquisition scores between the two groups? and (b) What difference exists in oral proficiency scores between the two groups? Two directional hypotheses predicted that pupils taught by specialist teachers would have higher mean scores on both measures. The sample consisted of 200 Grade 4 pupils selected through two-stage cluster sampling. One hundred pupils were taught by subject-specialist English teachers, and 100 pupils were taught by generalist classroom teachers. All pupils completed a 25-item vocabulary test (maximum score 50) and a speaking test with three tasks rated by two raters (maximum score 30). Data were analyzed using independent-samples t-tests with effect sizes (Cohen's d). The findings showed that pupils taught by subject-specialist English teachers scored significantly higher on both measures. For vocabulary scores, the specialist group (M = 38.67, SD = 4.79) outperformed the generalist group (M = 31.70, SD = 7.44), t(169.17) = 7.88, p < .001, one-tailed, d = 0.85. For speaking scores, the specialist group (M = 21.91, SD = 3.35) outperformed the generalist group (M = 17.47, SD = 3.42), t(198) = 9.28, p < .001, one-tailed, d = 1.00. Both null hypotheses were rejected. The findings are consistent with previous international research and support theoretical frxameworks including the Input Hypothesis, Output Hypothesis, and Interaction Hypothesis. The study makes unique contributions as the first empirical comparison of specialist and generalist teachers in Baghdad primary schools, using rigorous two-stage cluster sampling and reporting large effect sizes. Practical implications include recommendations for the Iraqi Ministry of Education to prioritize specialist English teachers in primary grades. Limitations include the absence of random assignment and pre-test measures.