The genus Dermocystidium comprises fungal pathogens of the order Dermocystida in the class Ichthyosporea (http://www.ncbi.nlm.nih.gov/taxonomy). Several different Dermocystidium species have been described, infecting a variety of fish hosts and producing gill infections, skin lesions and visceral disease all over the world (Wildgoose 1995; Pekkarinen & Lotman 2003; Pekkarinen et al. 2003; Feist et al. 2004; Zhang & Wang 2005). To our knowledge, there are two reports on Dermocystidium infections in cardinal tetra, Paracheirodon axelrodi and neon tetra, Paracheirodon innesi (Reichenbach-Klinke 1982; Lewisch 2010). However, these reports did neither include histological and ultrastructural examinations nor molecular genetic investigations to confirm the diagnosis and identify the aetiologic species. In January 2013, increased mortalities of cardinal tetra, P. axelrodi of a 350-L aquarium were reported, occurring after purchase of additional fish. During the clinical examination, most of the fish were swimming in normal active condition, but some were lethargic or displayed a transparent mass on the skin of the head or body. The masses were up to 5 mm in diameter and contained a central, white tubular structure. Similar masses were located on the fins of some fish, but were considerably smaller in these locations. A parasitic infestation of the skin of these fish was excluded via microscopic examination, and normal results were obtained after analysis of the water values and inspection of the pump and filter system. Six cardinal tetra, P. axelrodi and two firehead tetra, Hemigrammus bleheri were submitted for pathological examination in formalin and for molecular genetic examination in ethanol. Only two cardinal tetra displayed skin lesions. Before embedding, one tetra was post-fixed in Davidson’s fixative and sections of glycolmethacrylate/methylmethacrylate-embedded samples were routinely processed for histological examination and stained with haematoxylin and eosin (HE), Giemsa, silver impregnation and periodic acid Schiff (PAS) reaction according to standard protocols. Samples of the mass were also routinely processed for transmission and scanning electron microscopy on a transmission electron microscope (Zeiss EM 10) or on a digital scanning electron microscope (Zeiss DSM 950), respectively. The macroscopic examination of the cardinal tetra revealed a focally extensive, bulging, hemispherical, transparent oedema (4 mm in diameter) of the ventral skin on the head. Central, within Correspondence M C Langenmayer, Institute of Veterinary Pathology at the Centre for Clinical Veterinary Medicine, LMU Munich, Veterin€ arstrasse 13, 80539 Munich, Germany (e-mail: langenmayer@patho.vetmed.uni-muenchen.de) Langenmayer and Lewisch contributed equally to this work.