The Symphony No. 6 ("Fantaisies symphoniques"), H. 343, by Bohuslav Martinů was begun in New York City in 1951, after a hiatus of four years since its predecessor, and was tentatively completed three years later on 23 April 1953. In Paris, in the months following its completion, Martinů undertook some revisions to the score. It is dedicated to Charles Munch, on the occasion of the 75th anniversary of the Boston Symphony Orchestra, who premiered the symphony on 7 January 1955. Martinů originally called the work Fantaisies symphoniques, and this is sometimes regarded as its only correct title.

The symphony has three movements:

    I. Lento—Allegro—Lento
    II. Poco allegro (4/4)
    III. Lento

The symphony is quite unlike the previous five symphonies. In Paris in the autumn of 1955, Martinů described it to Miloš Šafránek as a work "without form. And yet something holds it together, I don't know what, but it has a single line, and I have really expressed something in it".

The Sixth Symphony is distinguished from its predecessors by a canzona-like structure, with an exceptionally high level of invention. Motivic development is carried out in series of extended sections each with a distinct texture, steadily increasing in speed through to the end of the second movement. The reversion in the finale to the lento tempo of the opening movement is accomplished more rapidly, with the correspondence becoming exact only close to the end.

The symphony opens with an hallucinatory, otherworldly texture that sounds like music only just in the process of being formed—music without rhythm, melody, or harmony. This is created using only nine instruments: three flutes, three trumpets, and three solo strings, in superimposed rhythmic layers that divide the slow beat into nine, ten, and twelve subdivisions simultaneously. The complexity produces what is in effect an aleatory texture, "a gateway into the imprecise realm of fantasy".