A freak of nature is a baroque opera with a dramatically engrossing plot. Such is the case with Montezuma, whose book was by Frederick the Great of Prussia. The music is reputed to be quite up to the above-average standards of the plot, as well. With extensive pruning, the Associate Artists Opera are presenting a rare experience.

Any chance to hear the Juillard Quartet should be considered seriously. Their program (Ives, Carter, Bartok) is the way concert-programming ought to be: with a definite audience in mind, avoiding attempts to please everyone at once.

Kirkland House continues its active musical season by presenting Robert Portney, one of the most gifted violinists ever to study at Harvard. His program has a number of spectaculars that should be attractive to those who are often scared off by solo violin recital repertoire.

Somehow the best undergraduate musicians find each other for chamber music. The quartet at Holmes Hall next Tuesday has some of the best players in the College. They are joined by Fred Fox, an excellent oboist who has remained at Harvard with Bryce-like tenacity.

SYMPHONY HALL. Ozawa conducting the BSO and Tanglewood Festival Chorus. Berlioz: Symphonie Fantastique and Lelio. Tickets: 266-1492. Feb. 15 (8:30 p.m.), 16, (2:00 p.m.), and 17 (8:30 p.m.).

CYCLORAMA, BOSTON CENTER FOR THE ARTS. Associate Artists Opera Company. Montezuma., an opera by Graun and Frederick the Great. Tickets: 542-0308. Feb. 16, 8 p.m.

SANDERS THEATER. Juilliard String Quartet. Quartets by Ives, Carter, and Bartok. Feb. 16, 8:30 p.m.

DUNSTER HOUSE LIBRARY. Songs for soprano and piano. Works by Cushman and Schumann. Feb. 17, 5:30 p.m.

KIRKLAND HOUSE JCR. Robert Portney, violin; Arlene Portney, piano. Works by Scarlatti, Chopin, Liszt, Franck, Wieniawski, and Bartok. Feb. 17, 8:30 p.m.

MUSEUM OF FINE ARTS: Lecture Hall. The Camerata. Musica Teutonica: German music during the Middle Ages and the Renaissance. Admission charge. Feb. 20, 7:15 p.m.

HOLMES HALL. Chamber music for oboe, strings, and piano. Works by Mozart, Bloch, and Britten. Feb. 20. 8 p.m.