This study analyzed the discourse of electronic mail (e-mail) exchanges between students of English as a second language (ESL) and other ESL learners from other cultures and at varying proficiency levels (keypals), focusing on what these exchanges may reveal about learners' progress through the "Zone of Proximal Development," a Vygotskian concept denoting the gap between what the learner can accomplish alone and what he can accomplish in cooperation with others who are more skilled or experienced. Subjects were approximately 150 intermediate ESL students at the University of Puerto Rico who use the computer laboratory once a week and regularly send and receive e-mail messages to/from each other or keypals in other countries. A variety of discourse modes were examined: expository; descriptive; narrative; and argumentative. Students were also surveyed concerning their perceptions of ESL learning through the experience. It was found that students were highly motivated to use e-mail for communication, and used peer tutoring and collaboration in composing outgoing messages and studying incoming ones. General chat sessions not focused on a particular topic appeared less productive for ESL learning. Excerpts from the student questionnaire are appended. (Contains 12 references.) (MSE)