The development of DNA-chip-technology has made it possible to analyse large numbers of probes in just one experiment. We show here the results of our first DNA-chip for the identification of phytoplankton. A set of previously published probes was chosen to be tested for their use on DNA-chips. Initially the probes were developed for other hybridization-techniques, such as dot-blot-hybridization or FISH (fluorescent in situ hybridization). The probes target the 18S-rDNA of phytoplankton at the level of division or class. With the resulting DNA-chip, it was possible to identify specifically Prymnesiophyta, Chlorophyta, and the heterokont algal class Bolidophyceae. Our results indicate that DNA-chip-technology has great potential in phytoplankton analysis. © 2004 J. Cramer in der Gebrüder Borntraeger Verlagsbuchhandlung.