The Cat Fanciers association lists the Boston fern (Nephrolepis exaltata--a.k.a. Sword Fern) as a non-toxic plant for cats.
Today, our little guy taught us differently by munching on my boston fern last night while we slept, and getting sick afterwards.
Non-toxic does not necessarily mean "edible". We all know what happens when cats eat things they aren't meant to swallow.
We took him to a veterinary ER, to make sure he would be OK. Apparently Boston Fern can cause gastrointestinal upset, lethargy, vomiting, loss of appetite, and diarrhea. We've seen all but the last one in our cat.
Cat's not happy about it either, but he seems to be recovering, getting his appetite back late this afternoon. (Right after we got home from the vet hospital, of course.)
From now on, only "edible" plants in the house.
Good thing I live in a semi-tropical climate, because my ferns are about to be banished to the warm shadiness of my back porch, where they will probably be just as happy anyway.
For a longer list of plants poisonous to your cat, see this list from the Cat Fanciers Association.