Colonial type variation of gonococci is well known, but change from type 4 to type 4 to type 1 is rare except in vivo. By observing quantitatively subcultures from a new liquid medium it was possible to follow the day-to-day changes in the ratio of colonial types present. The results obtained showed that type 1 colonies could be derived from type 5 inocula even in unsupplemented media. In unsupplemented liquid medium, type 4 inocula did not revert to other types and indeed colonial type 4 appeared to be the final form of the organism before it died out. If, however, iron in the form of ferric citrate was added to the medium, reversion occurred and type 1 colonies rapidly came to predominate.