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The trophozoites of a novel gregarine apicomplexan, Trichotokara nothriae n. gen. et sp., were isolated and characterized from the intestines of the onuphid tubeworm Nothria conchylega (Polychaeta), collected at 20 m depth from the North-eastern Pacific Coast. The trophozoites were 50-155 microm long with a mid-cell indentation that formed two prominent bulges (anterior bulge, 14-48 microm wide; posterior bulge, 15-55 microm wide). Scanning electron microscopy (SEM) demonstrated that approximately 400 densely packed, longitudinal epicytic folds (5 folds/microm) inscribe the surface of the trophozoites, and a prominently elongated mucron (14-60 microm long and 6-12 microm wide) was covered with hair-like projections (mean length, 1.97 microm; mean width, 0.2 microm at the base). Although a septum occurred at the junction between the cell proper and the mucron in most trophozoites, light microscopy (LM) demonstrated that the cell proper extended into the core of the mucron as a thin prolongation. A spherical nucleus (8-20 microm) was situated in the middle of the trophozoites, and gamonts underwent caudal syzygy. The small subunit (SSU) rDNA sequence and molecular phylogenetic position of T. nothriae was also characterized. The sequence from this species was the most divergent of all SSU rDNA sequences currently known from gregarines and formed a weakly supported clade with Lecudina polymorpha, which also possesses densely packed epicyctic folds (3-5 folds/mum) and a prominently elongated mucron., ((c) 2010 Elsevier Inc. All rights reserved.)