AbstractThe circumscription of the lichenized ascomycete genusIcmadophilaTrevis. in the familyIcmadophilaceaeTriebel was investigated. Sequences of the internal transcribed spacer (ITS) region and large subunit of nuclear ribosomal DNA (nuLSU) were generated for the fiveIcmadophilaspecies and additional members ofIcmadophilaceaefrom the generaDibaeis,Endocena,Knightiella,Siphula,SiphulellaandThamnolia. Phylogenetic analyses indicate that threeIcmadophilaspecies are closely related: the type,I. ericetorum(L.) Zahlbr., which is widespread in Eurasia and North America,I. aversa(Nyl.) Rambold & Hertel from Central and South America, andI. japonica(Zahlbr.) Rambold & Hertel, which is restricted to Far East Russia and Japan. The genusKnightiellaMüll. Arg. is reinstated to accommodateI. splachnirima(Hook.f. & Taylor) D. J. Galloway emend. L. Ludw., which occurs in New Zealand and Australia. Two further species ofKnightiella,K. eucalypti(Kantvilas) Kantvilas andK. queenslandicaKantvilas, are found to be unrelated toK. splachnirima, and are accommodated in two newly described genera,KnightiellastrumandSiphulopsisrespectively.KnightiellastrumL. Ludw. & Kantvilas is characterized by a squamulose, erhizinate, whitish to pale grey thallus with a green, coccoid photobiont and by containing thamnolic acid.SiphulopsisKantvilas & A. R. Nilsen is similarly characterized by an erhizinate, whitish to pale grey thallus, with a green, coccoid photobiont and containing thamnolic acid, but is instead fruticose. This study reveals considerable diversity within AustralasianIcmadophilaceae; ongoing work in the Southern Hemisphere and tropical regions may reveal additional species in this family and clarify the relationships of these newly described genera.