The family's last name The last name of the family has been revealed only once in the show, in the pilot episode, where Francis wears the name tag "Wilkerson" on his school uniform (it can be seen best in the scene where he is talking with his family on the phone). Also, though unaired, it appears in a joke from the original pilot script. In that script, Malcolm was walking to school when a neighborhood kid came running up shouting, "Malcolm, Malcolm, Malcolm. I was talking to my parents last night - I was listening to them talk, and what's your last name?" "Wilkerson, why?" Malcolm replied. "Oh. Who are the Pariahs?" said the other kid. The joke was eventually cut. One theory of why their last name has been played down is that the producers did not want them to be typecast as any particular ethnicity, and they likely hoped rerun viewers would miss Francis' name tag. A special feature on the series 1 DVD stated also that their surname was Wilkerson. A trailer on the UK channel Sky One in Early 2006 advised you to spend time with "The Simpsons" and "The Wilkersons", advertising Sunday night new episodes of both series. Another trailer was released to advertise the last episodes, of 24 and Malcolm in the Middle, it claimed Say Goodbye to the Wilkersons. However, on Bryan Cranston's official website, Mr. Cranston says the crew regularly joked amongst themselves that their last name is actually Nolastname. During the series finale, when Malcolm is being introduced for his graduation speech, his last name is not heard due to a squeak of the microphone. Just before Malcolm gives his graduation speech, Francis drops his employee ID on the ground and it clearly shows his name as "Francis Nolastname", this may be a joke or Francis is too embarrassed of his family. cited from wikipedia

Wilkerson or Nolastname depending on if you watch the 1st episode or the last one.