The three dimensional morphology of the thyroid tissue in juvenile rainbow trout Oncorhynchus mykiss was determined using serial transverse and longitudinal sections of lower jaws stained with haematoxylin and eosin. The thyroid tissue comprised interconnecting tubular units not the follicles that are widely described in the literature. Some of these tubules were subdivided by branching, giving the appearance of follicles in the sections. The thyroid tissue was aggregated into two major concentrations of tissue, associated with the first and second branchial arteries, with the majority of the tissue located dorsal and lateral to the aorta. These observations in juvenile rainbow trout suggest that thyroid tissue of some species of fishes may not be follicular, as previously assumed. The physiological and evolutionary implications have yet to be determined.