Someone told me recently that odd-lot orders can be very expensive on ECNs, i.e. Arca. He added that an odd-lot is any order that is not done in 100 share blocks. Ergo, a 401-share order would be considered an "odd-lot" and all 401 of those shares would be charged a hefty fee. My understanding was that when talking about stocks, an odd lot is any order under 100 shares, and so an order for, say, 750 shares would NOT be considered an odd-lot order, but 50 shares would be an odd-lot. If my friend is correct, if you get a partial fill on Arca, i.e. 692 shares on a 1000-share buy order, and then you try to sell those 692 shares, you're really getting screwed with that extra per-share fee, aren't you? I'd appreciate it if someone could clarify what an odd-lot is and what Arca charges for them. According to the Arca website, they charge $.003 per share, but they do not define what an odd lot is. I also did a search and found a quote that "Arca may charge as much as $.20 per share for odd-lot orders."