Abyss & Apex:
http://www.abyssapexzine.com/submissions/
Speculative fiction. Particularly interested in stories under 1500 words. Pays $.05/word. Note reading periods before submitting
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Electric Spec:
http://www.electricspec.com/submissions/submitting-fiction/
Speculative fiction. Not an exclusively flash fiction market, but will accept stories as short as 250 words. Pay is $20 per story, so the shorter the better as far as your per-word rate. Note issue-specific deadlines.
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Vestal Review:
http://vestalreview.net/About.html.
Publishes only flash fiction. Pays 3-10 cents/word. All genres except children's stories and hard SF.
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Nanoism:
http://nanoism.net/submit/.
This is a Twitter market. Stories cannot exceed 140 characters, or the maximum length of one tweet. Pay is $1.50 per story--that's a bit over a penny per character or roughly $.05/word. Prefers literary fiction and also accepts serial stories. Previously published tweets will be considered, but pay at $1/story
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For more flash fiction markets as well as additional information on the form, how to write it, and examples:
Flash Fiction Writers Market
http://www.flash-fiction-world.com/writers-market.html
or
Flash Fiction Chronicles
http://www.everydayfiction.com/flashfictionblog/flash-markets-2/[/size]