This paper describes a portable tracking device (TD) that records sequential, three-dimensional positions of an object as it moves through space. The operator manually tracks the object through a tripod-mounted, optical rangefinder at ranges of 250 to 5,000 m, and adjusts a thumbwheel to keep two images of the object in alignment. The rangefinder has a 1-meter base and rotates around horizontal and vertical axes. The rotations of the rangefinder and thumbwheel turn the shafts of three digital resolvers that connect to the parallel port of a portable microcomputer. The computer reads all three resolvers in 0.001 second, averages the data over 1 second, and stores the averages in memory. At the end of a track, the computer writes the data to a disk file for later analysis. The TD was designed to track birds (or similar objects) flying in natural conditions. The accuracy of the TD varies with range and depends on the skill of the operator. A point in space measured by the TD is the center of an approximately cylindrical confidence volume that contains the object 95% of the time. The length of the cylinder, measured along the cylinder axis, coincides with the line of sight of the rangefinder. An experienced operator can keep a flying bird within a confidence volume that is 3 m in diameter and 3 m long at a range of 250 m. As range increases, diameter and length increase in proportion to range, and range squared, respectively. The length of the confidence volume is 42 m at 1,000 m and 167 m at 2,000 m. © 1995 American Institute of Physics.