Controlled mating experiments demonstrating total reproductive isolation, together with consistent differences in morphological and cytological detail, confirmed the validity of the closely related group IX Ips: grandicollis, cribricollis, lecontei, montanus, confusus, paraconfusus, and hoppingi. The latter two species, new to science, were formerly considered to be confusus.Matings between grandicollis from eastern Canada and North Carolina were subnormally fertile; other intraspecific matings between individuals of the same population or distant populations produced fully fertile progeny. Interspecific pairings usually resulted in insemination and normal-appearing egg galleries, but, with one exception, none of the eggs hatched.One montanus and two confusus, each mated by males of the same species, produced all-daughter broods. This "sex-ratio" condition appeared to be the result of a matrilineally transferred cytoplasmic factor lethal to male embryos only.At first meiotic metaphase all species had the karyotypic formula 15 AA + Xyp, and only grandicollis was easily differentiated by bivalent configuration or size sequence. Most other species showed diagnostic differences at meiotic prophase or second meiotic metaphase. Pairing disruptions and anaphase bridges occurred in grandicollis interpopulational hybrids; meiosis was normal in progeny from interpopulational pairings of confusus, paraconfusus, and montanus.In grandicollis the median struts of the male genitalia were 1.5 times longer than the median lobe. The struts/lobe ratios for other species approximated 1.00 but differences for most comparisons between species were statistically significant. The shape of the pars stridens on females was different for most species and mean widths of striations thereon ranged from 0.517 μ for paraconfusus to 0.876 μ for lecontei. This character provided absolute discrimination of females of confusus from those of paraconfusus and hoppingi but the latter two species were most easily separated by differences in the density of punctures in the elytral declivity.It is hypothesized that contiguous allopatry and differences in host and ecological specialization, evidenced in species of group IX, is maintained by their high propensity for interspecific mating which fails completely to produce progeny.