Host and conspecific discrimination were tested as reproductive isolating mechanisms betweenIps paraconfusus Lanier infestingPinus coulteri (Torrey) andI. confusus (Le Conte) infestingP. monophylla (Fremont). In two areas (one area largely Coulter pine and the other largely pinyon pine) where these bark beetles and hosts cooccur in southern California, we induced pheromone production in host and nonhost logs using males of each species.Ips paraconfusus females joined both heterospecific and conspecific males tunneling in both tree species in each area.Ips confusus females failed to joinI. paraconfusus males in Coulter pine, but joinedI. paraconfusus in pinyon pine. Sympatry was demonstrated when females of both sibling species joined conspecific males in their respective hosts. Males attacked all four beetlehost treatment combinations in both areas. Laboratory tests confirmed these results. Males did not displace heterospecific males from nuptial chambers in hosts, and they did not occupy a gallery in which heterospecific males produced frass. Females that left conspecific males in a host were readily accepted by heterospecific males and oviposited in a nonhost.