Belfagor, commedia lirica in 1 prologue, 2 acts and 1 epilogue, music by Ottorino Respighi, Milan, Teatro alla Scala, 26 April 1923
Die versunkene Glocke (La campana sommersa), opera in 4 acts, music by Ottorino Respighi, Hamburg, Stadttheater, 18 November 1927
Odette, opera in 3 acts, music by Mario Marangolo, Brescia, Teatro Grande, 1929[1]
Maria egiziaca , mistero lirico in 1 act and three episodes, music by Ottorino Respighi, New York City, Carnegie Hall, 16 March 1932
La fiamma, melodramma in 3 acts, music by Ottorino Respighi, Rome, Teatro dell'Opera, 23 January 1934
Lucrezia , opera in 1 act, music by Ottorino Respighi, Milan, Teatro alla Scala, 24 February 1937
Gli Orazi, opera (istoria per musica) in 1 act, music by Ennio Porrino, Milan, Teatro alla Scala, 1 February 1941[1][2]
Guastalla was also the author of the subject of Respighi's ballet Belkis, regina di Saba (1932) and collaborated with Respighi for a revision (1934) of the libretto of L'Orfeo of Claudio Monteverdi.[2]