An electrophoretical and immunological study is made of nine species of pycnogonids, representing seven families, from the Catalan coast. An electrophoretogram of each species is given and the antigenic properties of its protein bands are determined. Taking as comparative basis the serological affinities of the different species with Ammothella longipes and by morphological considerations as well, five major evolutionary lines have been established. The Nymphonidae are considered the more primitive. The Ammotheidae — Tanystylidae are considered a central group by their greater number of protein bands and their greater capacity of morphological diversification. The Phoxichilidiidae-Endeidae are considered intermediate between the former and the Callipallenidae and Pycnogonidae, the latter two lines are considered the most evolved.