ABSTRACT: This systematic study documents the taxonomy, diversity, and distribution of 40 species of the predominately Californian trapdoor spider genus Aptostichus Simon, 1891. Thirty-three of these species are newly described: Aptostichus dantrippi, Aptostichus cabrillo, Aptostichus pennjillettei, Aptostichus asmodaeus, Aptostichus nateevansi, Aptostichus chiricahua, Aptostichus icenoglei, Aptostichus isabella, Aptostichus muiri, Aptostichus barackobamai, Aptostichus sinnombre, Aptostichus hedinorum, Aptostichus aguacaliente, Aptostichus chemehuevi, Aptostichus sarlacc, Aptostichus derhamgiulianii, Aptostichus anzaborrego, Aptostichus serrano, Aptostichus mikeradtkei, Aptostichus edwardabbeyi, Aptostichus killerdana, Aptostichus cahuilla, Aptostichus satleri, Aptostichus elisabethae, Aptostichus f
