Efraín Rivera Castillo (May 25, 1924 – March 12, 1978), widely known as Mon Rivera, was a prominent Puerto Rican bandleader celebrated for his contributions to plena, salsa, and Latin jazz. Born in Mayagüez, Puerto Rico, he was the son of Monserrate "Don Mon" Rivera Alers, a revered plena composer. Don Mon's socially conscious and humorous plenas, such as Aló, ¿Quién Ñama?, influenced Efraín’s musical journey.
Efraín initially gained local fame as a shortstop for the Indios de Mayagüez before pursuing a career in music. A multi-instrumentalist, he performed with various orchestras, popularizing his father's plenas and composing his own. His groundbreaking use of an all-trombone brass section became a defining element of Afro-Rican orchestra music and later influenced salsa pioneers like Willie Colón.
In 1963, Efraín released Que gente averiguá, showcasing his innovative plena-salsa fusion. Struggles with addiction and health issues sidelined him in the 1960s, but a collaboration with Willie Colón on the 1975 album Se Chavó El Vecindario revitalized his career, producing hits like Ya Llegó.
Efraín passed away in 1978 in New York City, leaving a lasting legacy in plena and salsa. His innovative trombone sound endures in modern Latin music.
