ABSTRACT The din phenomenon, first discussed in the literature of second language acquisition by Krashen, is a form of mental rehearsal in which words, sounds, and phrases of the second language are spontaneously replayed in the mind. This paper reports the results of a survey on the din phenomenon in which 52 Spanish-speaking ESL college students participated. The researcher was able to determine that the din was a very common phenomenon among the participants, to gain new insights into the nature of the din, and to establish that the din is present at almost all stages of acquisition. An important discovery was that mental rehearsal may be voluntary as well as spontaneous, voluntary rehearsal appearing as a way of monitoring language. The pedagogical implications and research possibilities of the din phenomenon and mental rehearsal are examined.