The Biological Station on Helgoland has one of the longest data series for phytoplankton species composition in the world. Since 1962, phytoplankton has been counted to a species level on a daily basis (weekdays). One of the main reasons why this data set has rarely been used is that it has never been subjected to proper quality control or inventoried properly. This paper describes the quality control which we have now carried out on this data set and also represents the first inventory of the data and its meta-information. A comparison of the electronic database where the data is archived and the original recorded lists from the different persons dealing with the samples was an integral part of the evaluation. Apart from recording inconsistencies in species identification, we also recorded and corrected the many differences found between the electronic and paper data. The evaluation was carried out in a direct comparative manner to discern taxonomic discrepancies, and in order to find random errors in the transfer of data from paper to computer random checks of the data were carried out. This paper serves to illustrate the level of quality control required when dealing with archived phytoplankton data and illustrates the typical problems encountered. © Springer-Verlag and AWI 2004.