Soak the limas in cold water for 5 to 6 hours. Drain off this water and add fresh water until beans are covered. Bring to a boil, then reduce heat to low and cook for 30 minutes until tender.
Melt the butter and add the sugar, then salt and pepper to taste. Turn the beans into a greased casserole, leaving about 1/4 to 1/3 cup of cooking liquid. Pour butter and seasoning over beans. Lay strips of bacon on top. Cover and bake for 1 hour at 350 degrees. Add more cooking liquid while baking if beans seem too dry.
Remove cover and put marshmallows on top of the bacon. Brown marshmallows under the broiler. Serve in the casserole.