Improvisation is regarded as the most sublime element in the jíbaro folk music tradition of Puerto Rico. This tradition originated by the jíbaro, the simple rural farmer of Puerto Rico's heartland, involves the complicated art of improvising in décima, a ten-line poetic form, as well as improvisation of melodic lines played on the cuatro, a small guitar-like instrument. Since jíbaro improvisation is an art that is transmitted orally and involves a seemingly spontaneous act, it might seem odd to talk about a theory of improvisation within this style of music. My ethnographic research however has revealed that improvisation in jíbaro music is actually a highly structured performance practice and involves an informal theory that is based on the knowledge of archetypal patterns that generate and organize jíbaro improvisations.Recent theories of music which establish parallels between music, language, and cognition (Lerdhal and Jackendoff; Clarke; Gjerdingen) have lead me to believe that improvisation in jíbaro music is generated by the combination of archetypal patterns that create a musical syntax. These patterns are stored in minds of jíbaro performers as cognitive schemas. My study is also based on the work of Puerto Rican scholars Luis M. Alvarez and Angel Quintero who have identified African rhythmic patterns as the generative musical source in many styles of Puerto Rican folk music. By combining theories of music and ethnographic methods, this paper will provide a greater understanding of orally transmitted cultural expressions, which utilize improvisation, as well as give insight to the cognitive processes that shape this performance practice.