This article reports the findings of the Peer-Led Team Learning (PLTL) model intervention study in an introductory biology course at a Puerto Rican private university. PLTL introduces to the traditional class format an additional workshop session in which students interact in small groups to solve challenging exercises under the guidance of a peer leader. The questions to be answered are: (1) Does PLTL advance the understanding and learning of biology concepts?; and (2) Does PLTL increase the number of students passing the course? The methodology consisted of a quantitative approach comparing differences between control (non-PLTL) and experimental (PLTL) groups. Data collection included: final grades, pretests/posttests, opinion questionnaires, and postsession quizzes. The analysis showed an increase in the percent of successful final grades and quizzes, and less dropouts of PLTL over non-PLTL groups. A significant gain of learning was observed between pretest and posttest of both groups (P[less than or equal to]0.05), with no difference among groups. Students' questionnaires resulted in a higher percentage of positive opinion for the PLTL model. In conclusion, PLTL is a positive educational model for students who struggle to succeed in their introductory science courses.