Deep convolutional neural networks are emerging as the state of the art method for supervised classification of images also in the context of taxonomic identification. Different morphologies and imaging technologies applied across organismal groups lead to highly specific image domains, which need customization of deep learning solutions. Here we provide an example using deep convolutional neural networks (CNNs) for taxonomic identification of the morphologically diverse microalgal group of diatoms. Using a combination of high-resolution slide scanning microscopy, web-based collaborative image annotation and diatom-tailored image analysis, we assembled a diatom image database from two Southern Ocean expeditions. We use these data to investigate the effect of CNN architecture, background masking, data set size and possible concept drift upon image classification performance. Surprisingly, VGG16, a relatively old network architecture, showed the best performance and generalizing ability on our images. Different from a previous study, we found that background masking slightly improved performance. In general, training only a classifier on top of convolutional layers pre-trained on extensive, but not domain-specific image data showed surprisingly high performance (F1 scores around 97%) with already relatively few (100-300) examples per class, indicating that domain adaptation to a novel taxonomic group can be feasible with a limited investment of effort. Diatoms are microscopic algae possessing silicate shells called frustules 1. They inhabit marine and freshwater environments as well as terrestrial habitats. Taxonomic composition of their assemblages is routinely assessed using light microscopy in ecological, bioindication and paleoclimate research 2-4. Silicate frustules cleaned of organic material and embedded into high refractive index mountant on cover slips represent the most widely used type of microscopic preparations for such analyses 5,6. Attempts to computerize parts or the whole of this workflow have been made repeatedly, starting with Cairns 7 , and in the most complete manner so far by the ADIAC project 8 , motivated by the desire to speed up the taxonomic enumeration process, to reduce its dependence on highly trained taxonomic experts, and to make identification results more reproducible and transparent. Recently, we described an updated re-implementation of most parts of this workflow, covering high throughput microscopic imaging, segmentation and outline shape feature extraction of diatom specimens 9,10 which we since mainly applied in morphometric investigations 11-14. The missing component in this workflow has been automated or computer-assisted taxonomic identification. For this purpose, i.e. image classification in a taxonomic context, deep convolutional neural networks (CNNs) are currently becoming the state-of-the-art technique. Due to the broadening availability of high throughput, in part, in situ, imaging platforms 15-18 , and large publicly available image sets 19 , marine plankton has pro...