BCD, actually, given that Fortran dates from the mid-1950s. EBCDIC only appeared more or less around Fortran IV, in the early 1960s. Many printers in those days had a 48-character chain/train. After upper-case letters, digits, and a few essential punctuation marks (like . and ,), you weren't left with many options. The 60-character set of PL/I was a luxury back then, let alone lower case.
