Summary(1) The occurrence of haemoglobin in Phaenocora typhlops (Vejdovsky) is reported.(2) Rhabdocoels were examined spectrophotometrically. The distribution of the pigment was studied by a sodium nitroprusside‐benzidine technique, and the distribution of ferric iron with Perl's technique.(3) Characteristic bands of oxyhaemoglobin were observed at 540 and 580 nm. The oxyhaemoglobin was reduced to haemoglobin by the addition of sodium dithionite and an absorption band appeared at about 560 nm.(4) Haematin was found principally around the pharynx, though a little was found in front of the latter and along the length of the gut. Ferric iron was distributed throughout the worm.(5) The occurrence of haemoglobin in P. typhlops and P. unipunctata is compared. It is suggested that less occurs in the former species. Whether or not this is related to the presence of zoochlorellae is considered. Certainly both species burrow actively in mud where reduced oxygen conditions can occu