In Mississippi, all qualified parties nominate by primary. However, a primary is not actually held for parties with no primary contests. Because the ballot-qualified minor parties in Mississippi never have contests in their primaries, they in effect have already nominated for 2010. Primary date in Mississippi in 2010 is June 1.
The Reform Party has U.S. House nominees in all four Mississippi districts. The Libertarian Party has nominees in two districts, and the Constitution Party has one nominee. See this story.
U.S. House is the only partisan office up in Mississippi this year. There is no U.S. Senate race up, and state elections are held in the odd years before presidential election years, i.e, 2011. Thanks to Steve Rankin for the link.