The effect of ESL (English as a Second Language) teaching techniques on student learning was investigated. A group of 18 ESL teachers and their classes were observed across four lessons having a similar content. Students were pre- and post-tested over a six-month period in two measures of language performance: oral comprehension and production. Teaching behaviors, such as asking guided questions, correcting grammatical structures, explaining new vocabulary, and teacher's knowledge of linguistics, were found to influence student growth positively, while a rapid pace and an exaggerated use of modeling were found to have negative effects. The effects associated with these teaching behaviors held for student growth measured in terms of either oral comprehension or production. Regression analysis of selected teaching behaviors accounted for two-thirds of the explained variance in student learning. (Author/AM)