Philip José Farmer, a science fiction writer who shocked readers in the 1950s by depicting sex with aliens and who went on to challenge conventional pieties of the genre in caustic fables set on bizarre worlds of his own devising, died Wednesday. He was 91 and lived in Peoria, Illinois.

His official web site announced his death, saying he “passed away peacefully in his sleep.”

Mr. Farmer’s distinctive blend of intellectual daring and pulp-fiction prose found a worldwide audience. His more than 75 books have been translated into 22 languages and published in more than 40 countries.

Though he wrote many admired short stories, he was best known for his multi-novel series. These sprawling, episodic works gave him room to explore every nuance of a provocative premise while indulging his taste for lurid, violent action.

In his Riverworld series Mr. Farmer imagined a river millions of miles long on a distant planet where virtually everyone who has died on Earth is physically reborn and given a second chance to make something of life. In his Dayworld series, Earth’s overpopulation crisis has been relieved by a technical fix; each person spends one day of the week awake and the other six days in suspended animation. In his World of Tiers series, mad demigods create pocket universes for their own amusement, only to face rebellion from their putative creatures.