The number of available e-book titles is growing
The book, first published in 1883, is widely held to be the German philosopher's most engaging and readable work.
In it, he sets out many of the theories for which he is best known, including the idea of the so-called superman, and famously stated: "God is dead."
The UK-based company behind the venture, AudiobooksForFree.Com (ABFF), is banking on MP3 technology to help Nietzsche's aphorisms find an echo in the 21st Century.
The company manages to supply the books free by finding and using out-of-copyright versions.
Managing director Ruslan G Fedorovsky acknowledged that, at first sight, Zarathustra might not seem a likely basis for a blockbusting audiobook.
But he said the audience for audiobooks on the internet was "not just MP3 hackers", and that the signs were that the book would do well.
A two-minute extract of the audiobook, available for downloading on the Napster site before that closed, had been a great success, he said.
"Right away, Zarathustra became astonishingly popular among the MP3 community and has already swept through the majority of popular P2Ps post-Napster file-sharing networks."
'Commuters'
"Our listener base is very, very broad," Mr Fedorovsky told BBC News Online.
Commuters formed the biggest single group using audiobooks, he said.
But they were also used by foreign students learning English in the UK and abroad, UK ex-pats abroad and blind and visually impaired people.
"We are very grateful to the Friedrich Nietzsche Society and personally to their leader, Dr Duncan Large, for help in arranging this important production. We could not have done it without their assistance," he added.
Despite the German philosopher's unexpected popularity as an audiobook, Nietzsche is not apparently expected to unseat ABFF's current top-selling title.
The company's most downloaded audiobook "by far", is a far gentler offering - Jerome K Jerome's Three Men in a Boat.