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Millipedes are known from body fossils as early as the Silurian, and they are an important part of modern global soil ecosystems. Little is known, however, of the morphology of millipede burrows in either the modern or the fossil record. The burrowing behavior and traces of two species of extant millipedes were studied in a laboratory setting. The goal of this research was to determine the connections between millipede morphology, burrow morphology, and media conditions. Specimens of Archispirostreptus gigas and Orthoporus ornatus were placed in large, sediment-filled terrariums. The sediment was varied in terms of texture, compactness, and moisture. Traces produced by the millipedes were then cast and described. The burrow morphology of each species was primarily controlled by trace-making behavior, including excavation methods and burrow occupation time. Orthoporus ornatus burrowed by excavation to construct subvertical shafts leading to terminal chambers occupied for several days to weeks. Archispirostreptus gigas burrowed by sediment compression to construct large-diameter, sinuous tunnels occupied for hours to days. Increasing the clay content, compaction, and moisture content of the sediment together served to inhibit burrowing. Specimens of A. gigas were unable to burrow into compact or clay-rich sediments, whereas specimens of O. ornatus were able to burrow into even firm clay. Neither species was able to burrow into water-saturated media. The study described in this paper aids in understanding the relationships among the morphology of terrestrial biogenic structures, organism size, organism behavior, and media conditions.