The life history of the shield-backed bug, Pachycoris stallii Uhler (Heteroptera: Scutelleridae), immatures was studied on its host plant, Croton californicus Muell.-Arg. (Euphorbiaceae), in Baja California Sur, Mexico. Immature stages are described and illustrated. Pachycoris stallii is bi- or multivoltine and occurs in xeric areas with sandy soil where it is rarely encountered away from C. californicus. Nymphs and adults feed on seeds within C. californicus fruit. Bugs oviposit on the underside of leaves, and females guard their eggs and first-instar nymphs from natural enemies. Embryonic orientation of prolarvae is nonrandom; each embryo is oriented with its venter directed toward the ground. This orientation may facilitate aggregation of first instars. The longitudinal axes of eggs are always oriented upward at about a 16° angle of deviation from a line perpendicular to the leaf surface. This is the first recorded observation of this phenomenon in Pentatomoidea. Experimental removal of females guarding first instars results in 100% loss of nymphs, and this is attributed to disruption of the aggregative behavior of nymphs. Maternal guarding appears to be a net benefit to P. stallii, despite possible costs to the brooding female.