Studies were conducted on the biology and behavioral ecology of sympatric populations of spider wasps, Pepsis cerberus and P. mexicana from the Chihuahuan Desert. Females selectively hunt the theraphosid spider Aphonoplema steindachneri as their host. Adult body size was related to the size of the host spider that larval wasps fed upon. Mean body length and head capsule width for males and females of P. cerberus was 18.9 and 21.9 mm, respectively, as compared to 19.8 and 22.8 for P. mexicana . There were no significant interspecific differences in fecundity, and fecundity was related to female body size. Number of eggs in ovarioles ranged from 5 to 26 (P. cerberus) and 4 to 20 (P. mexicana). Mean values for length (mm), width (mm) and weight (mg) of eggs for P. cerberus were 4.25 mm, 1.31 mm, and 7.21 mg, as compared to 4.31, 1.23, and 7.14 for P. mexicana. Both species exhibited five larval instars (L1–L5). Under laboratory conditions (22 ± 0.5°C, 60–70% relative humidity), 1st-instar larvae of both species emerged within 3.2–4.1 days after oviposition. The amount of time required from oviposition to cocoon formation for P. cerberus and P. mexicana ranged from 19 to 22 and 20 to 24 days, respectively. The duration of the pupal stage ranged from 14 to 18 days. First-instar larvae (L1) fed initially on the cuticular lamellae and underlying connective tissues in the area of the host's book lungs, then proceeding to medial regions of the midgut, and finally to the heart and cerebral ganglion ingested. Peak period of activity for P. cerberus and P. mexicana were between 1,000 to 1,359 and 1,400 to 1,759, respectively. Adults of each species exhibited differences in the plants visited to obtain nectar.