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 developled for other hybridisation-techniques, such as dot-blot-hybridisation or FISH (fluorescent in situ hybridisation). The probes target the 18S-DNA of phytoplankton at the level 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 to serve as a useful tool for the phytoplankton analysis.