In the US, do ultralight aircraft have tail numbers? Just wondering if ultralight aircraft in the United States have tail numbers like the larger general aviation aircraft have?
Assuming that you mean an ultralight as defined in part 103, there is no requirement to be registered or have a tail number. 14 CFR 103.7(c) says:


  Notwithstanding any other section pertaining to registration and marking of aircraft, ultralight vehicles are not required to be registered or to bear markings of any type