The nesting success of Xylocopa californica arizonensis Cresson (Hymenoptera:Anthophoridae) is determined primarily by the availability of pollen and nectar and by the presence of a source of nest substrate. X. c. arizonensis appears to depend upon the pollen of Larrea tridentata or Prosopis spp. for larval food, when it is available. Predators, parasites and nest competitors are relatively unimportant in limiting nesting success.