Arriaga soon became a teaching assistant in Fétis's class, noted and highly praised both by fellow students and other faculty at the Conservatoire for his talent. Cherubini referred to Arriaga's fugue for eight voices (lost) based on the "Credo ... et vitam venturi" simply as "a masterpiece", and Fétis was no less effusive — apparently, what impressed all his mentors was his use of sophisticated harmonies, counterpoint and fugue with minimal or no formal instruction. Fétis was already familiar with Arriaga's now-lost opera "Los Esclavos Felices" ("The Happy Slaves"), stating that "without any knowledge whatsoever of harmony, Juan Crisóstomo wrote a Spanish opera containing wonderful and completely original ideas." Arriaga was well supported during his four years in Paris by his father, but the intensity of his commitment to his studies at the Conservatoire and his meteoric rise, based on his teachers' compliments and assessments of his promise, may have taken a toll on his health: he died in Paris ten days before his twentieth birthday, of a lung ailment (possibly tuberculosis), or exhaustion, perhaps both. He was buried in an unmarked grave at the "Cimetière du Nord" in Montmartre. Thanks to the Spanish Embassy, since 1977 there has been a plaque marking the house at 314 rue Saint-Honoré in memory of the composer.