The dimensions, manufacture and application of a mini-borer to take 3-mm cores are described. This new instrument seems particularly well-suited to determining ages of saplings in situations where collection of cross-sections and coring with standard increment borers are too destructive, and where node-counting may not be sufficiently accurate. The design has resulted in trouble-free use in the field, and the mini-cores can be stored in the straws, and mounted and sanded in the core mounts used for collections of the standard 4–5-mm increment cores.