I ordered these for my husband to wear under his dress pants. He recently had surgery and gets chilled easily. These are nice and light and allows him to wear his regular slacks and still keep warm without overheating. When ordering, I couldn't find whether they had a fly or not --they do have a fly opening!
My husband bought a pair of these pants after not finding the silk pair (also LLBean) in stock at an area retail LL Bean. He liked the silk ones so much that he was reluctant to try them, but he has since enjoyed them to the point where I just ordered another pair for him, and two for my son. They are happy with how weightless, warm, and moisture repellent they are...I'm happy with their durability. The silk pair have all developed holes along the sides, and weren't nearly as warm.
