Morphometric measurements were made on various gill components of different stages in the life cycle of the anadromous parasitic lamprey, L. fluviatilis, and its nonparasitic derivative L. planeri. The total gill area, expressed in terms of body weight, of both larval (1462-2717 mm2 g-1) and adult (1402-2337 mm2g-1) L. fluviatilis were greater than those previously recorded in the literature on lamprey gill measurements and were comparable with those found in the most active teleosts. The gills of the 2 Lampetra species were apparently identical in the larval stages and those of metamorphosing and adult L. planeri were similar to those of metamorphosing L. fluviatilis. Although the pharyngeal arrangement of lampreys differed greatly from that of teleosts, there were many features of the gills indicative of convergence between the 2 groups. In a given stage in the life cycle of lampreys, the secondary lamellae on either side of the filaments also alternated, became more widely spaced as the filament length increased and increased in area as the body weight became greater. The fractional cumulative increase in secondary lamellae area along a line following the presumed direction of water flow was also represented by a sigmoid curve. While at metamorphosis the pharynx became considerably modified to accommodate the change from a unidirectional to a tidal respiratory water flow, the total gill areas of the ammocoete were similar to those of metamorphosing stages which attained adult characteristics. There were clearly differences in some of the components that influenced and contributed toward the total gill area. In terms of body weight, the number and total length of the filaments and the total number of secondary lamellae, together with the number of secondary lamellae found on a given distance of filament, were greater in late metamorphosing stages, while the reverse was true for the average bilateral area of the secondary lamellae which was considerably greater in ammocoetes.