In this study, the author investigated factors underlying vocal improvisation achievement and relationships with the singers' musical background. Participants were 102 college students in Australia and the United States who performed 3 jazz improvisations and 1 free improvisation. Jazz improvisations were rated on rhythmic, tonal, and creative thinking criteria; free improvisations were rated only on creativity criteria. The results are as follows: (a) A significant difference was found between jazz and free improvisation achievement; (b) extensive jazz experience, especially study and listening, was found to be significantly correlated with vocal improvisation achievement; (c) 3 factors were found to underlie jazz improvisation: jazz syntax, vocal creativity, and tonal musicianship; and (d) 3 factors were found to underlie free improvisation: musical syntax, vocal creativity, and scat syllable creativity. (Contains 4 tables and 1 note.)