The haemoglobins of six different species of trematodes: Gastrothylax crumenifer, Srivastavaia indica, Gigantocotyle explanatum, Fasciolopsis buski, Gastrodiscoides hominis, Isoparorchis hypselobagri and their three different hosts: Bubalus bubalis, Sus scrofa, and Wallago attu were spectrophotometrically investigated, and were found to contain porphyrin IX as the common prosthetic group. Oxyhaemoglobin, carbonmonoxy-haemoglobin and reduced haemoglobin of all 6 species of trematodes and their 3 hosts under study gave similar absorption maxima. Distinct differences were, however, observed in the nature of the spectral curves of cyanmethaemoglobin which exhibit 2 absorption maxima in the beta and the alpha region in the case of all trematodes whereas in the case of similar host haemoglobin derivatives only one single broad peak in the 536-540 nm region was obtained. With respect to pyridine derivatives all the trematode haemoglobins show a sharp peak in the alpha region and a minor hump in the beta region except Gastrothylax crumenifer. All three host pyridine haemoglobin derivatives show only a single broad peak at 570 nm.