Spiders and pseudoscorpions on old pedunculate oaks (Quercus robur) with tree cavities were studied in a Central European floodplain (South Moravia, Czech Republic). Altogether 322 specimens from 47 spider taxa and 71 specimens of six pseudoscorpion species were collected during 2010 and 2011 from tree cavities using two methods. More specimens and species of spiders were obtained from flight interception traps and more specimens and species of pseudoscorpions were obtained from pitfall traps. Remarkable records represent typical cavity dwellers, i.e. the spider Midia midas (Simon, 1884), the pseudoscorpions Larca lata (Hansen, 1884) and Apocheiridium ferum (Simon, 1879), the latter occurs mostly under tree bark. Five arachnid species are listed in the Czech red list: Midia midas, Leptorchestes berolinensis (C. L. Koch, 1846), Dipoena erythropus (Simon, 1881), Larca lata and Dendrochernes cyrneus (L. Koch, 1873).