Chelicerates are an ancient arthropod group with a distinct body plan composed of an anterior (prosoma) and a posterior portion (opisthosoma). The expression of the Hox gene Ultrabithorax (Ubx) has been examined in a single representative of the chelicerates, the spider Cupiennius salei. In spiders, Ubx expression starts in the second opisthosomal segment (O2). Because the first opisthosomal segment (O1) in spiders is greatly reduced relative to other chelicerates, we hypothesized that the observed Ubx expression pattern might be secondarily modified. Shifts in the anterior boundary of the expression of Ubx have been correlated with functional shifts in morphology within malacostracan crustaceans. Thus, the boundary of Ubx expression between chelicerates with different morphologies in their anterior opisthosoma could also be variable. To test this prediction, we examined the expression patterns of Ubx and abdominal-A (collectively referred to as UbdA) in two basal chelicerate lineages, scorpions and xiphosurans (horseshoe crabs), which exhibit variation in the morphology of their anterior opisthosoma. In the scorpion Paruroctonus mesaensis, the anterior border of early expression of UbdA is in a few cells in the medial, posterior region of the O2 segment, with a predominant expression in O3 and posterior. Expression later spreads to encompass the whole O2 segment and a ventral, posterior portion of the O1 segment. In the xiphosuran Limulus polyphemus, early expression of UbdA has an anterior boundary in the segment. Later in development, the anterior boundary moves forward one segment to the chilarial (O1) segment. Thus, the earliest expression boundary of UbdA lies within the second opisthosomal segment in all the chelicerates examined. These results suggest that rather than being derived, the spider UbdA expression in O2 likely reflects the ancestral expression boundary. Changes in the morphology of the first opisthosomal segment are either not associated with changes in UbdA expression or correlate with late developmental changes in UbdA expression.