In this paper, the nature of the fine interlocking acicular ferrite microstructure in HSLA steel weld metals is investigated. The results strongly suggest the acicular ferrite is comprised of intragranularly nucleated Widmanstatten ferrite. Further, it is shown that the active nucleation sites for this ferritic product are weld metal inclusions. Sympathetic nucleation then takes place which leads eventually to the fine, interlocking microstructure which is a characteristic of acicular ferrite.