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We compared the morphological characteristics of the scale covers and bodies of 2 economically important scale species, California red scale, Aonidiella aurantii (Maskell), and oleander scale, Aspidiotus nerii (Bouche), to determine their protective value against predation by the coccinellid Rhyzobius lophanthae (Blaisdell). We also compared the morphology of the mandibles of R. lophanthae with that of Chilocorus cacti (L.), a specific predator of diaspidid scales. A higher percentage of R. lophanthae larvae survived when they fed on 2nd-and 3rd-instar A. aurantii and all stages of A. nerii than when they fed on gravid and parturient A. aurantii. Adult R. lophanthae took longer to consume 3rd-instar A. aurantii than 3rd-instar A. nerii and were frequently unable to penetrate the scale body of gravid and parturient A. aurantii. Predator preference for scale stage and species reflected the ease with which it was able to penetrate the scale cover and body of its prey. The hardened scale cover of gravid and parturient A. aurantii when coupled with the sclerotized scale body provided the scale with substantial immunity to predation by R. lophanthae for approximately two-thirds of the scale life span. We conclude that R. lophanthae is unlikely to suppress A. aurantii populations below those of economic concern when it is the sole biological control agent present. In contrast, the entire life span of A. nerii is vulnerable to predation, and this species appears to be suppressed to low densities by R. lophanthae.