Data were collected for 45 characters from all of the published descriptions of 73 genera of living pycnogonids. Each genus was compared over all characters. equally weighted, with every other genus and similarity values were calculated by means of a restricted version of Gower's Generalized Coefficient (SG). The resulting matrix of intergeneric similarity values was subjected to principal co‐ordinate analysis followed by single nearest‐neighbour analysis at 0.5% SGintervals of the data as described by the first two vectors.Four groupings, at 82.5, 81.5 80.5 and 78.5 SG, were well defined. From this pattern was developed the hypothesis of a classification of the genera into 5 orders containing 30 families.Achelia, Ascorhynchus, Boreonymphon, PentacolossendeisandPhoxichilidium.amongst some other genera, were found to lie in unexpected systematic positions.Rhynchothoraxwas found to be widely different from all other genera.New names are proposed for certain taxa and descriptions are given of suprageneric groups.Further analysis is required to discern character state correlations within the proposed polythetic groups, so that simple keys can be constructed.The researches were supported in part by Scientific Investigations Grants administered by the Royal Society.The computations of the comparisons and the analysis were performed on the ICL 4.70 computer at Rothamsted Experimental Station and I offer my grateful thanks to Mr G. J. S. Ross of that institution for much useful advice and for guiding my data speedily through the system.Without the data there could have been no computations. Through her painstaking help and encouragement in the gathering of data from the multitude of notes and publications, my wife Patricia ensured that the study was accomplished.Without the pycnogonids and their devotees there could have been no data. My debt to scores of pycnogonid‐enthusiasts for their discoveries is self‐evident. However, without Joel Hedgpeth's example and his persisting and friendly encouragement, the joys of pycnogonid studies would have el