HAEMOGLOBIN dissolved in the blood plasma has been recorded in all the major divisions of entomostracan Crustacea except one1. This respiratory blood pigment has been found in all species of Phyllopoda examined, in most Cladocera, in a few Ostracoda, in some harpacticid and parasitic Copepoda and in some Rhizocephala, but not hitherto in the Branchiura. This is a small group of so-called fish lice, which includes the well-known Argulus foliaceus (L.). I have been unable to detect haemoglobin in the pale blood of species of this genus, but Dolops ranarum (Stuhlmann) has red blood and I find that it shows strong absorption bands of oxyhaemoglobin. The animals were fouhd parasitizing the siluroid fish Bagrus docmac (Forskal) in Lake Victoria, and I was able to examine them, thanks to the staff of the East African Fisheries Research Laboratory at Jinja, Uganda.