We have completed constructing and begun operating the Mark-II drift chamber vertex detector. The chamber, based on a modified jet cell design, achieves 30 μm spatial resolution and <1000 μm track-pair resolution in pressurized CO 2 gas mixtures. Special emphasis has been placed on controlling systematic errors, including the use of novel construction techniques which permit accurate wire placement. Chamber performance has been studied with cosmic ray tracks collected with the chamber located both inside and outside the Mark-II. Results on spatial resolution, average pulse shape, and some properties of CO 2 mixtures are presented.