ABSTRACTThe haptophyte symbiont Chrysochromulina andersonii sp. nov. isolated from the polycystine radiolarian Dictyocoryne truncatum was examined using both natural and cultured materials by light, scanning and transmission electron microscopy as well as molecular phylogenetic analyses of the small subunit (SSU) and large subunit (LSU) rDNA sequences. In culture, the motile cells were saddle-shaped, 3–5 µm long and 3–4 µm wide. They possessed two equal to subequal flagella, a coiling haptonema, and two parietal golden-brown chloroplasts. The scales were arranged in two layers composed of two types: scales with flat rims, 0.19–0.26 µm in diameter, formed the inner layer; whereas, scales with upright rims, 0.25–0.45 µm in diameter, formed the outer layer. Both scales had a faint cross in the centre of the scale face, and the radiating ridges extended from the faint cross to the periphery. Molecular phylogenetic analysis indicated that the SSU and LSU rDNA sequences from our symbiont fell within the clade of Chrysochromulina. However, our symbiont differed in the details of the patterning on the scale face from the other species of Chrysochromulina. Thus, we propose C. andersonii sp. nov. for the symbiotic alga from the polycystine radiolarian D. truncatum.