The Grosser Arbersee (UTM-grid-reference: UQ 63) is located at an altitude of 934 m on the slope of the Mt. Grosser Arbet (alt. 1456 m) in the Bayrischer Wald near the border between the German Federal Republic and Czechoslovakia. The lake is 500 m long (E-W) and 150-250 m wide (N-S). On sunny summer days its shores are crowded with visitors. The water of the lake is clear, acid (pH 4,5) and poor in electrolytes (0,5° dH). Pine woods reach down to the stony banks with their scarce reed vegetation. Submerged vegetation is lacking, except for floating Sphagnum, intermixed with Carex rostrata, C. limosa, Caltha palustris, Menyanthes trifoliata, and, in the inlets, some Nuphar luteum in the western part of the lake. On August 9, 1978, from 9.00-16.00, the following dragonfly species were observed: Enallagma cyathigerum (Charp.), flying in small numbers over the Sphagnum and the open water; Aeshna juncea (L.), many specimens, also copulating pairs; A. subarctica Walk., some 66 hunting over the Sphagnum, 1 9 ovipositing; A. cyanea (Mull.), several 66 and 99, some ovipositing at the woody bank near the Sphagnum; Cordulia aenea (L.), 1 6 hunting at the inlet, 1 dead 9; Somatochlora metallica (Vander L.), many all around the lake; Sympetrum danae (Sulz.), several specimens, some emerging, others ovipositing; S. vulgatum (L.), 1 subadult, 1 9; Leucorrhinia dubia (Vander L.), several specimens flying over the Sphagnum, some ovipositing.