Six months after they separated, "Little House on the Prairie" actress Melissa Gilbert has filed for divorce from actor Bruce Boxleitner ... TMZ has learned.

According to documents filed with L.A. County Superior Court, Gilbert blamed the split on irreconcilable differences ... and she's asking for joint legal and physical custody of their son Michael.

Melissa and Bruce married in 1995 ... but separated back in March. They have one child together ... and each have adult children from previous marriages.

http://www.tmz.com/2011/08/25/melissa-gilbert-bruce-boxleitner-divorce-marriage-separation-split-break-up-little-house-on-the-prairie-tron-divorce-custody-joint/

How sad...I thought they'd worked everything out. :(

It's always sad when a marriage comes to an end. :(

Really sad that so few "Hollywood" marriages really endure anymore -- even the breakups that should come as a surprise oftentimes don't, because we've seen so many over the years.